Mastering Date and Time in C#

A Complete Guide for .NET Developers

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Table of Contents

1. Why Date and Time Are Tricky in C#
2. Introduction to DateTime and DateTimeKind
3. Working with DateTime.Now vs DateTime.UtcNow
4. Formatting Dates and Times
5. Parsing Dates and Times
6. Time Zones in .NET
7. Dealing with Daylight Saving Time
8. Measuring Time and Durations
9. Working with DateOnly and TimeOnly in .NET 6+
10. Using TimeProvider in .NET 8+
11. Best Practices for Storing Dates in Databases

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Chapter 1: Why Date and Time Are Tricky in C

Hey there! If you've ever shipped a feature only to have users complain that "the timestamps are all wrong" or spent hours debugging why your scheduled job ran at the wrong time, you're in good company. Date and time handling is one of those deceptively complex topics that trips up even experienced developers.

Think about it: when a user in Tokyo schedules a meeting for 3 PM tomorrow, and your server is hosted in AWS US-East, what time should your application actually store? When your logging system shows "2024-03-15 14:30:00" but doesn't specify the timezone, how do you know if that's local server time, UTC, or the user's timezone ?

Real-World Scenarios Where DateTime Causes Pain

Global SaaS Applications: Your customer in Sydney reports that their daily report runs at 2 AM instead of 8 AM. The culprit? Your code used DateTime.Now on the server instead of converting to the user's local timezone.

// Bad - Server time, confuses users globally
var reportTime = DateTime.Now.AddHours(12); 

// Good - User's timezone
var reportTime = TimeZoneInfo.ConvertTime(DateTime.UtcNow.AddHours(12), userTimeZone);

Financial Systems: A billing system charges customers twice during daylight saving time transitions because it doesn't handle the "ambiguous hour" properly.

Audit Logging: Security logs show inconsistent timestamps because some use local time and others use UTC, making incident investigation nearly impossible.

API Integrations: Your REST API returns dates in different formats depending on the server's culture settings, breaking client applications.

Why C# Makes This Challenging

C#'s DateTime struct is powerful but has some design quirks that can bite you. It carries a Kind property that's often ignored, leading to timezone confusion. The default DateTime.Now seems convenient but often causes more problems than it solves in modern applications.

The good news? Modern .NET has introduced better tools like DateOnly, TimeOnly, and TimeProvider that address many of these issues. This book will teach you when and how to use them effectively.

What You'll Learn

By the end of this book, you'll confidently handle dates and times in any .NET application. You'll know when to use UTC vs local time, how to properly store dates in databases, and how to write testable code that doesn't break when daylight saving time kicks in.

Chapter Exercise: Think of a bug or confusion you've encountered with dates/times in your applications. Write it down - we'll revisit it at the end of the book to see how the techniques you learn could have prevented it.

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Chapter 2: Introduction to DateTime and DateTimeKind

Before we dive into the nitty-gritty of date manipulation, let's understand the foundation: the DateTime struct and its often-overlooked Kind property. Understanding DateTimeKind is crucial because it determines how your DateTime values behave in timezone conversions and comparisons.

The Three DateTimeKind Values

Every DateTime instance has a Kind property that can be one of three values :

• DateTimeKind.Local: Represents local time on the current system
• DateTimeKind.Utc: Represents Coordinated Universal Time (UTC)
• DateTimeKind.Unspecified: The timezone is unknown or not relevant

var localTime = DateTime.Now;           // Kind = Local
var utcTime = DateTime.UtcNow;          // Kind = Utc  
var unspecified = new DateTime(2024, 3, 15, 14, 30, 0); // Kind = Unspecified

Console.WriteLine($"Local: {localTime.Kind}");      // Local
Console.WriteLine($"UTC: {utcTime.Kind}");          // Utc
Console.WriteLine($"Manual: {unspecified.Kind}");   // Unspecified

Why DateTimeKind Matters

The Kind property affects how DateTime behaves in several important ways :

Timezone Conversions: Only DateTime values with Local or Utc kinds can be converted between timezones safely.

var localTime = DateTime.Now;
var utcTime = DateTime.UtcNow;
var unspecified = new DateTime(2024, 3, 15, 14, 30, 0);

// These work fine
var localToUtc = localTime.ToUniversalTime();
var utcToLocal = utcTime.ToLocalTime();

// This throws an exception!
try 
{
    var badConversion = unspecified.ToUniversalTime(); // InvalidOperationException
}
catch (ArgumentException ex)
{
    Console.WriteLine("Can't convert Unspecified DateTime to UTC!");
}

Equality Comparisons: Two DateTime values with different Kind properties are never equal, even if they represent the same moment.

var utc = new DateTime(2024, 3, 15, 14, 0, 0, DateTimeKind.Utc);
var local = new DateTime(2024, 3, 15, 14, 0, 0, DateTimeKind.Local);

Console.WriteLine(utc == local);        // False, even if they're the same time!
Console.WriteLine(utc.Equals(local));   // Also False

Best Practices for DateTimeKind

Always specify Kind when creating DateTime: Instead of relying on the default Unspecified, be explicit :

// Bad - Kind is Unspecified
var meetingTime = new DateTime(2024, 3, 15, 14, 30, 0);

// Good - Explicitly UTC
var meetingTimeUtc = new DateTime(2024, 3, 15, 14, 30, 0, DateTimeKind.Utc);

// Better - Use DateTime.SpecifyKind if you have an unspecified DateTime
var specified = DateTime.SpecifyKind(meetingTime, DateTimeKind.Utc);

Check Kind before timezone operations: Always verify the Kind before converting timezones to avoid runtime exceptions.

public static DateTime ConvertToUtc(DateTime dateTime)
{
    return dateTime.Kind switch
    {
        DateTimeKind.Utc => dateTime,
        DateTimeKind.Local => dateTime.ToUniversalTime(),
        DateTimeKind.Unspecified => throw new ArgumentException(
            "Cannot convert unspecified DateTime to UTC"),
        _ => throw new ArgumentOutOfRangeException()
    };
}

Working with DateTimeOffset Instead

For many scenarios, consider using DateTimeOffset instead of DateTime. It includes timezone offset information, eliminating ambiguity :

// DateTimeOffset includes timezone offset
var nowWithOffset = DateTimeOffset.Now;           // 2024-03-15 14:30:00 -04:00
var utcWithOffset = DateTimeOffset.UtcNow;        // 2024-03-15 18:30:00 +00:00

// No ambiguity in comparisons
var offset1 = new DateTimeOffset(2024, 3, 15, 14, 30, 0, TimeSpan.FromHours(-4));
var offset2 = new DateTimeOffset(2024, 3, 15, 18, 30, 0, TimeSpan.Zero);
Console.WriteLine(offset1 == offset2); // True - same moment in time

Common Mistakes with DateTimeKind

Ignoring Kind in database scenarios: Storing DateTime with Unspecified kind in databases often leads to timezone confusion later.

Mixing different Kinds: Comparing or sorting DateTime values with different Kind properties without considering timezone differences.

Assuming Local means user's timezone: In web applications, "local" means the server's timezone, not the user's.

Chapter Summary: Understanding DateTimeKind is essential for proper date/time handling. Always be explicit about timezone context, prefer DateTimeOffset when timezone information matters, and validate Kind before timezone operations.

Chapter Exercise: Create a method that takes a DateTime parameter and returns a string describing its Kind and whether it can be safely converted to UTC. Test it with DateTime.Now, DateTime.UtcNow, and a manually created DateTime.

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Chapter 3: Working with DateTime.Now vs DateTime.UtcNow

This might be the most important chapter in the book. The choice between DateTime.Now and DateTime.UtcNow affects every timestamp in your application, and getting it wrong leads to bugs that are painful to track down and even harder to fix in production.

The Fundamental Problem with DateTime.Now

DateTime.Now returns the current date and time in the system's local timezone. In your development environment, this might seem perfect. But in real applications, this creates several problems :

Server Location Dependency: Your web application's timestamps depend on where your server is physically located.

// This code behaves differently in different hosting environments
public class OrderService
{
    public Order CreateOrder(Customer customer)
    {
        return new Order
        {
            CustomerId = customer.Id,
            CreatedAt = DateTime.Now  // Problem: Depends on server location!
        };
    }
}

If your server moves from US-East to EU-West, all new timestamps change timezone, breaking reports and confusing users.

Cloud Environment Issues: Modern cloud platforms can migrate your containers or serverless functions across different regions, changing the "local" timezone unexpectedly.

// In AWS Lambda or Azure Functions, you can't rely on consistent timezone
public class LoggingService
{
    public void LogError(string message)
    {
        var timestamp = DateTime.Now; // Could be any timezone!
        Console.WriteLine($"{timestamp}: ERROR - {message}");
    }
}

Why DateTime.UtcNow Is Usually Better

UTC (Coordinated Universal Time) is timezone-neutral and consistent worldwide. When you use DateTime.UtcNow, your application behaves the same regardless of server location :

public class OrderService
{
    public Order CreateOrder(Customer customer)
    {
        return new Order
        {
            CustomerId = customer.Id,
            CreatedAt = DateTime.UtcNow  // Always consistent, regardless of server location
        };
    }
}

Benefits of UTC:

• Consistent across all environments
• No daylight saving time complications
• Easier database storage and querying
• Simplified timezone conversions

Real-World Scenarios: When Each is Appropriate

Use DateTime.UtcNow for:

• Database timestamps
• API responses
• Logging and auditing
• Scheduling and job processing
• Any data that will be shared across timezones

// Good examples of UTC usage
public class AuditLogger
{
    public void LogUserAction(string action, int userId)
    {
        var auditEntry = new AuditEntry
        {
            Action = action,
            UserId = userId,
            Timestamp = DateTime.UtcNow  // Consistent for global audit trails
        };

        _database.AuditEntries.Add(auditEntry);
    }
}

Use DateTime.Now for:

• Displaying current time to users (with proper timezone conversion)
• Local file naming or logging where server timezone is relevant
• Calculations that need to account for local business hours

// Example where local time might be appropriate
public class LocalFileManager
{
    public string GenerateLogFileName()
    {
        var localTime = DateTime.Now;
        return $"app_log_{localTime:yyyyMMdd_HHmmss}.txt";  // Server-local filename
    }
}

Performance Considerations

DateTime.UtcNow is actually faster than DateTime.Now because it doesn't need to perform timezone conversion :

// Performance test example
public class PerformanceComparison
{
    public void ComparePerformance()
    {
        var sw = Stopwatch.StartNew();

        // DateTime.Now (slower - needs timezone conversion)
        for (int i = 0; i < 1000000; i++)
        {
            var now = DateTime.Now;
        }
        sw.Stop();
        Console.WriteLine($"DateTime.Now: {sw.ElapsedMilliseconds}ms");

        sw.Restart();

        // DateTime.UtcNow (faster - no conversion needed)  
        for (int i = 0; i < 1000000; i++)
        {
            var utcNow = DateTime.UtcNow;
        }
        sw.Stop();
        Console.WriteLine($"DateTime.UtcNow: {sw.ElapsedMilliseconds}ms");
    }
}

Converting UTC to User's Local Time

When you need to display UTC timestamps to users, convert them to the user's timezone at the presentation layer :

public class TimeDisplayService
{
    public string FormatTimeForUser(DateTime utcTime, TimeZoneInfo userTimeZone)
    {
        // Always start with UTC, then convert for display
        var userLocalTime = TimeZoneInfo.ConvertTimeFromUtc(utcTime, userTimeZone);
        return userLocalTime.ToString("yyyy-MM-dd HH:mm:ss zzz");
    }
}

// Usage example
var order = orderService.CreateOrder(customer);  // Stores UTC
var userTimeZone = TimeZoneInfo.FindSystemTimeZoneById("Eastern Standard Time");
var displayTime = timeDisplayService.FormatTimeForUser(order.CreatedAt, userTimeZone);

Common Anti-Patterns to Avoid

Storing Local Time in Database: Never store DateTime.Now directly in databases without timezone context.

// Bad - timezone information lost
public void SaveEvent(Event eventData)
{
    eventData.CreatedAt = DateTime.Now;  // Which timezone?
    _db.Events.Add(eventData);
}

// Good - always UTC in database
public void SaveEvent(Event eventData)
{
    eventData.CreatedAt = DateTime.UtcNow;  // Always UTC
    _db.Events.Add(eventData);
}

Mixing UTC and Local Times: Don't mix DateTime values with different Kind properties without explicit conversion.

// Dangerous - comparing different timezone kinds
var localTime = DateTime.Now;
var utcTime = DateTime.UtcNow;

// This comparison might not work as expected
if (localTime > utcTime.AddHours(-5))  // Problematic
{
    // Logic might be wrong depending on actual timezone offset
}

Chapter Summary

Use DateTime.UtcNow as your default choice for most application scenarios. It provides consistency, better performance, and eliminates timezone-related bugs. Reserve DateTime.Now for specific cases where server-local time is genuinely required.

Chapter Exercise: Refactor a class that currently uses DateTime.Now to use DateTime.UtcNow instead. Add a method to convert stored UTC times to a specific user timezone for display. Consider how this change affects your existing code and data.

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Chapter 4: Formatting Dates and Times

Once you're consistently using UTC internally, you'll need to present dates and times to users in readable, localized formats. C# provides powerful formatting options, but with great power comes great complexity. Let's master the art of making dates and times user-friendly.

Standard Date and Time Format Strings

C# provides standard format strings that handle most common scenarios :

var utcNow = DateTime.UtcNow;  // 2024-03-15 18:30:45

Console.WriteLine(utcNow.ToString("d"));  // 3/15/2024 (short date)
Console.WriteLine(utcNow.ToString("D"));  // Friday, March 15, 2024 (long date)
Console.WriteLine(utcNow.ToString("t"));  // 6:30 PM (short time)  
Console.WriteLine(utcNow.ToString("T"));  // 6:30:45 PM (long time)
Console.WriteLine(utcNow.ToString("f"));  // Friday, March 15, 2024 6:30 PM (full short)
Console.WriteLine(utcNow.ToString("F"));  // Friday, March 15, 2024 6:30:45 PM (full long)
Console.WriteLine(utcNow.ToString("g"));  // 3/15/2024 6:30 PM (general short)
Console.WriteLine(utcNow.ToString("G"));  // 3/15/2024 6:30:45 PM (general long)

Custom Format Strings for Precise Control

When standard formats aren't enough, custom format strings give you complete control :

var dateTime = new DateTime(2024, 3, 15, 18, 30, 45, DateTimeKind.Utc);

// Custom formats for different use cases
Console.WriteLine(dateTime.ToString("yyyy-MM-dd"));           // 2024-03-15 (ISO date)
Console.WriteLine(dateTime.ToString("yyyy-MM-dd HH:mm:ss")); // 2024-03-15 18:30:45
Console.WriteLine(dateTime.ToString("MMM dd, yyyy"));        // Mar 15, 2024
Console.WriteLine(dateTime.ToString("dddd, MMMM dd"));       // Friday, March 15
Console.WriteLine(dateTime.ToString("HH:mm"));               // 18:30 (24-hour)
Console.WriteLine(dateTime.ToString("h:mm tt"));             // 6:30 PM (12-hour)

Critical Format Characters

Understanding these format characters helps you build exactly the format you need :

var dt = new DateTime(2024, 3, 5, 8, 7, 6);

// Year formats
Console.WriteLine(dt.ToString("yy"));    // 24 (2-digit year)
Console.WriteLine(dt.ToString("yyyy"));  // 2024 (4-digit year)

// Month formats  
Console.WriteLine(dt.ToString("M"));     // 3 (month number)
Console.WriteLine(dt.ToString("MM"));    // 03 (zero-padded month)
Console.WriteLine(dt.ToString("MMM"));   // Mar (abbreviated month)
Console.WriteLine(dt.ToString("MMMM"));  // March (full month name)

// Day formats
Console.WriteLine(dt.ToString("d"));     // 5 (day number)
Console.WriteLine(dt.ToString("dd"));    // 05 (zero-padded day)
Console.WriteLine(dt.ToString("ddd"));   // Tue (abbreviated weekday)
Console.WriteLine(dt.ToString("dddd"));  // Tuesday (full weekday name)

// Time formats
Console.WriteLine(dt.ToString("h"));     // 8 (12-hour, no leading zero)
Console.WriteLine(dt.ToString("hh"));    // 08 (12-hour, zero-padded)
Console.WriteLine(dt.ToString("H"));     // 8 (24-hour, no leading zero)
Console.WriteLine(dt.ToString("HH"));    // 08 (24-hour, zero-padded)

Culture-Aware Formatting

Different cultures format dates differently. Using CultureInfo ensures your application respects user preferences :

using System.Globalization;

var dateTime = new DateTime(2024, 3, 15, 18, 30, 0);

// US format (MM/dd/yyyy)
var usCulture = CultureInfo.GetCultureInfo("en-US");
Console.WriteLine(dateTime.ToString("d", usCulture)); // 3/15/2024

// European format (dd/MM/yyyy)
var ukCulture = CultureInfo.GetCultureInfo("en-GB");  
Console.WriteLine(dateTime.ToString("d", ukCulture)); // 15/03/2024

// German format 
var germanCulture = CultureInfo.GetCultureInfo("de-DE");
Console.WriteLine(dateTime.ToString("d", germanCulture)); // 15.03.2024

// Japanese format
var japaneseCulture = CultureInfo.GetCultureInfo("ja-JP");
Console.WriteLine(dateTime.ToString("d", japaneseCulture)); // 2024/03/15

Formatting for APIs: ISO 8601

For APIs and data exchange, use ISO 8601 format for consistency and interoperability :

public class ApiResponseFormatter
{
    public string FormatForApi(DateTime utcDateTime)
    {
        // ISO 8601 format with UTC indicator
        return utcDateTime.ToString("yyyy-MM-ddTHH:mm:ss.fffZ");
        // Output: 2024-03-15T18:30:45.123Z
    }

    public string FormatWithOffset(DateTimeOffset dateTime)
    {
        // ISO 8601 with timezone offset
        return dateTime.ToString("yyyy-MM-ddTHH:mm:ss.fffzzz");  
        // Output: 2024-03-15T14:30:45.123-04:00
    }
}

Practical Formatting Helpers

Create utility methods for common formatting scenarios :

public static class DateTimeFormatters
{
    public static string ForUserDisplay(DateTime utcDateTime, TimeZoneInfo userTimeZone)
    {
        var localTime = TimeZoneInfo.ConvertTimeFromUtc(utcDateTime, userTimeZone);
        return localTime.ToString("MMM dd, yyyy 'at' h:mm tt");
        // Output: Mar 15, 2024 at 2:30 PM
    }

    public static string ForLogFile(DateTime utcDateTime)
    {
        return utcDateTime.ToString("yyyy-MM-dd HH:mm:ss.fff 'UTC'");
        // Output: 2024-03-15 18:30:45.123 UTC
    }

    public static string ForFileName(DateTime dateTime)
    {
        // Safe for filenames - no problematic characters
        return dateTime.ToString("yyyyMMdd_HHmmss");
        // Output: 20240315_183045
    }

    public static string RelativeTime(DateTime utcDateTime)
    {
        var diff = DateTime.UtcNow - utcDateTime;

        return diff.TotalDays switch
        {
            < 1 when diff.TotalHours < 1 => $"{(int)diff.TotalMinutes} minutes ago",
            < 1 => $"{(int)diff.TotalHours} hours ago", 
            < 7 => $"{(int)diff.TotalDays} days ago",
            _ => utcDateTime.ToString("MMM dd, yyyy")
        };
    }
}

Common Formatting Mistakes

Hardcoding formats: Don't assume one format works globally :

// Bad - assumes US format
public string GetDateDisplay(DateTime date)
{
    return date.ToString("MM/dd/yyyy");  // Confusing for international users
}

// Good - respects user's culture
public string GetDateDisplay(DateTime date, CultureInfo culture)
{
    return date.ToString("d", culture);  // Adapts to user's preference
}

Ignoring timezone context: Always consider whether you're formatting UTC or local time :

// Dangerous - unclear what timezone this represents
public string FormatTimestamp(DateTime timestamp)
{
    return timestamp.ToString("yyyy-MM-dd HH:mm:ss");  // Missing timezone context!
}

// Better - explicit about timezone
public string FormatTimestamp(DateTime utcTimestamp, TimeZoneInfo displayTimeZone)
{
    var localTime = TimeZoneInfo.ConvertTimeFromUtc(utcTimestamp, displayTimeZone);
    return localTime.ToString("yyyy-MM-dd HH:mm:ss zzz");  // Includes timezone info
}

Chapter Summary

Master both standard and custom format strings, always consider culture when formatting for users, and use ISO 8601 for APIs. Create reusable formatting utilities to maintain consistency across your application.

Chapter Exercise: Build a DateTimeDisplayService class with methods for formatting dates in different contexts: user display (with culture), API responses (ISO 8601), log files, and relative time ("2 hours ago"). Test with different cultures and timezones.

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Chapter 5: Parsing Dates and Times

If formatting is about output, parsing is about input - and input is where things get messy. Users type dates in various formats, APIs send different date strings, and configuration files contain dates in yet other formats. Robust parsing prevents your application from crashing when it encounters unexpected date formats.

The Basic Parsing Methods

C# offers several methods for parsing date strings, each with different behaviors and use cases :

// Parse - throws exception on invalid input
try
{
    var date1 = DateTime.Parse("3/15/2024");        // Works in US culture
    var date2 = DateTime.Parse("15/3/2024");        // Might fail in US culture
    var date3 = DateTime.Parse("2024-03-15");       // Usually works
}
catch (FormatException ex)
{
    Console.WriteLine($"Parse failed: {ex.Message}");
}

// TryParse - returns bool, safer approach
if (DateTime.TryParse("3/15/2024", out DateTime result))
{
    Console.WriteLine($"Parsed successfully: {result}");
}
else
{
    Console.WriteLine("Failed to parse date");
}

Culture-Specific Parsing

Different cultures interpret date formats differently. Always consider the source culture when parsing :

using System.Globalization;

var dateString = "15/03/2024";

// Parse with US culture (MM/dd/yyyy) - this will fail!
var usCulture = CultureInfo.GetCultureInfo("en-US");
if (DateTime.TryParse(dateString, usCulture, DateTimeStyles.None, out DateTime usResult))
{
    Console.WriteLine($"US format: {usResult}");
}
else
{
    Console.WriteLine("Failed to parse as US format"); // This will execute
}

// Parse with UK culture (dd/MM/yyyy) - this works!
var ukCulture = CultureInfo.GetCultureInfo("en-GB");
if (DateTime.TryParse(dateString, ukCulture, DateTimeStyles.None, out DateTime ukResult))
{
    Console.WriteLine($"UK format: {ukResult}"); // March 15, 2024
}

Exact Parsing with ParseExact

When you know the exact format, use ParseExact for better control and performance :

// ParseExact requires exact format match
var dateString = "2024-03-15 18:30:45";
var format = "yyyy-MM-dd HH:mm:ss";

try
{
    var exactDate = DateTime.ParseExact(dateString, format, CultureInfo.InvariantCulture);
    Console.WriteLine($"Parsed exactly: {exactDate}");
}
catch (FormatException)
{
    Console.WriteLine("Format doesn't match exactly");
}

// TryParseExact for safer exact parsing
if (DateTime.TryParseExact(dateString, format, CultureInfo.InvariantCulture, 
    DateTimeStyles.None, out DateTime exactResult))
{
    Console.WriteLine($"Exact parse succeeded: {exactResult}");
}

Handling Multiple Formats

Real applications need to handle various date formats from different sources :

public static class FlexibleDateParser
{
    private static readonly string[] DateFormats = 
    {
        "yyyy-MM-dd",                 // ISO 8601 date
        "yyyy-MM-ddTHH:mm:ss",       // ISO 8601 datetime
        "yyyy-MM-ddTHH:mm:ss.fff",   // ISO 8601 with milliseconds
        "yyyy-MM-ddTHH:mm:ss.fffZ",  // ISO 8601 UTC
        "MM/dd/yyyy",                 // US format
        "dd/MM/yyyy",                 // European format
        "yyyy/MM/dd",                 // Alternative format
        "MMM dd, yyyy",               // Mar 15, 2024
        "dd-MMM-yyyy",                // 15-Mar-2024
        "yyyyMMdd"                    // Compact format
    };

    public static DateTime? ParseFlexible(string dateString)
    {
        if (string.IsNullOrWhiteSpace(dateString))
            return null;

        // Try each format until one works
        foreach (var format in DateFormats)
        {
            if (DateTime.TryParseExact(dateString.Trim(), format, 
                CultureInfo.InvariantCulture, DateTimeStyles.None, out DateTime result))
            {
                return result;
            }
        }

        // If exact formats fail, try general parsing
        if (DateTime.TryParse(dateString, CultureInfo.InvariantCulture, 
            DateTimeStyles.None, out DateTime generalResult))
        {
            return generalResult;
        }

        return null; // Could not parse
    }
}

Parsing ISO 8601 Dates from APIs

APIs commonly use ISO 8601 format. Handle both UTC and offset formats :

public static class ApiDateParser
{
    public static DateTime? ParseApiDate(string isoDateString)
    {
        if (string.IsNullOrWhiteSpace(isoDateString))
            return null;

        // Handle common ISO 8601 variations
        var formats = new[]
        {
            "yyyy-MM-ddTHH:mm:ss.fffZ",      // UTC with milliseconds
            "yyyy-MM-ddTHH:mm:ssZ",          // UTC without milliseconds  
            "yyyy-MM-ddTHH:mm:ss.fffzzz",    // With timezone offset
            "yyyy-MM-ddTHH:mm:sszzz",        // With timezone offset, no ms
            "yyyy-MM-ddTHH:mm:ss"            // No timezone info
        };

        foreach (var format in formats)
        {
            if (DateTime.TryParseExact(isoDateString, format, CultureInfo.InvariantCulture,
                DateTimeStyles.RoundtripKind, out DateTime result))
            {
                return result;
            }
        }

        return null;
    }
}

DateTimeStyles: Important Parsing Options

The DateTimeStyles enum provides important options for parsing behavior :

var dateString = "2024-03-15T18:30:45Z";

// Different DateTimeStyles behaviors
var styles = DateTimeStyles.None;
DateTime.TryParse(dateString, CultureInfo.InvariantCulture, styles, out DateTime result1);
Console.WriteLine($"None: {result1} (Kind: {result1.Kind})");

styles = DateTimeStyles.AssumeUniversal;
DateTime.TryParse(dateString, CultureInfo.InvariantCulture, styles, out DateTime result2);
Console.WriteLine($"AssumeUniversal: {result2} (Kind: {result2.Kind})");

styles = DateTimeStyles.RoundtripKind;  // Preserves Kind information
DateTime.TryParse(dateString, CultureInfo.InvariantCulture, styles, out DateTime result3);
Console.WriteLine($"RoundtripKind: {result3} (Kind: {result3.Kind})");

Common Parsing Pitfalls

Assuming single format: Don't assume all date strings follow one format :

// Bad - assumes MM/dd/yyyy format
public DateTime ParseUserDate(string input)
{
    return DateTime.ParseExact(input, "MM/dd/yyyy", CultureInfo.InvariantCulture);
    // Fails for international users or different formats
}

// Good - handles multiple formats gracefully
public DateTime? ParseUserDate(string input, CultureInfo userCulture)
{
    // First try with user's culture
    if (DateTime.TryParse(input, userCulture, DateTimeStyles.None, out DateTime result))
        return result;

    // Fallback to flexible parsing
    return FlexibleDateParser.ParseFlexible(input);
}

Ignoring timezone information: When parsing, consider whether the result should be UTC or local :

// Problematic - loses timezone context
public DateTime ParseApiTimestamp(string timestamp)
{
    return DateTime.Parse(timestamp); // Kind might be Unspecified!
}

// Better - explicit timezone handling
public DateTime ParseApiTimestamp(string timestamp)
{
    if (DateTime.TryParse(timestamp, CultureInfo.InvariantCulture, 
        DateTimeStyles.RoundtripKind, out DateTime result))
    {
        return result.Kind == DateTimeKind.Unspecified 
            ? DateTime.SpecifyKind(result, DateTimeKind.Utc)
            : result;
    }
    throw new FormatException($"Cannot parse timestamp: {timestamp}");
}

Validation After Parsing

Always validate parsed dates make sense in your business context :

public static class DateValidator
{
    public static DateTime? ParseAndValidateBirthDate(string dateString)
    {
        var parsed = FlexibleDateParser.ParseFlexible(dateString);

        if (!parsed.HasValue)
            return null;

        var date = parsed.Value;

        // Validate business rules
        if (date > DateTime.Today)
        {
            Console.WriteLine("Birth date cannot be in the future");
            return null;
        }

        if (date < new DateTime(1900, 1, 1))
        {
            Console.WriteLine("Birth date seems unreasonably old");
            return null;
        }

        return date;
    }
}

Chapter Summary

Use TryParse methods over Parse to avoid exceptions, always consider culture when parsing user input, use ParseExact when you know the format, and implement flexible parsing for robust applications. Always validate business rules after successful parsing.

Chapter Exercise: Create a ConfigurationDateParser class that can parse dates from configuration files, supporting multiple formats (ISO 8601, US format, European format). Include validation to ensure dates are reasonable and add logging for parsing failures.

---

Chapter 6: Time Zones in .NET

Time zones are where date/time programming gets really complex. Different regions observe different UTC offsets, daylight saving time rules vary by location and change over time, and political decisions can modify timezone rules. Let's master .NET's timezone handling so your global applications work correctly.

Understanding TimeZoneInfo

The TimeZoneInfo class is your gateway to proper timezone handling in .NET. It provides information about timezone offsets, daylight saving rules, and conversion capabilities :

// Get system timezone information
var localTimeZone = TimeZoneInfo.Local;
Console.WriteLine($"Local timezone: {localTimeZone.DisplayName}");
Console.WriteLine($"Standard name: {localTimeZone.StandardName}");  
Console.WriteLine($"Current offset: {localTimeZone.GetUtcOffset(DateTime.Now)}");

// Get specific timezone by ID
var easternTimeZone = TimeZoneInfo.FindSystemTimeZoneById("Eastern Standard Time");
var pacificTimeZone = TimeZoneInfo.FindSystemTimeZoneById("Pacific Standard Time");
var londonTimeZone = TimeZoneInfo.FindSystemTimeZoneById("GMT Standard Time");

Console.WriteLine($"Eastern: {easternTimeZone.DisplayName}");
Console.WriteLine($"Pacific: {pacificTimeZone.DisplayName}");  
Console.WriteLine($"London: {londonTimeZone.DisplayName}");

Finding Available Time Zones

Different operating systems have different timezone IDs. Windows uses different IDs than Linux/macOS :

public static class TimeZoneFinder
{
    public static void ListAllTimeZones()
    {
        var timeZones = TimeZoneInfo.GetSystemTimeZones();

        foreach (var tz in timeZones.Take(10)) // Show first 10
        {
            Console.WriteLine($"ID: {tz.Id}");
            Console.WriteLine($"Display: {tz.DisplayName}");
            Console.WriteLine($"Standard: {tz.StandardName}");
            Console.WriteLine("---");
        }

        Console.WriteLine($"Total timezones available: {timeZones.Count}");
    }

    public static TimeZoneInfo FindTimeZoneByName(string partialName)
    {
        var timeZones = TimeZoneInfo.GetSystemTimeZones();
        return timeZones.FirstOrDefault(tz => 
            tz.DisplayName.Contains(partialName, StringComparison.OrdinalIgnoreCase) ||
            tz.Id.Contains(partialName, StringComparison.OrdinalIgnoreCase));
    }
}

Converting Between Time Zones

The core functionality is converting UTC times to local timezone and vice versa :

public static class TimeZoneConverter
{
    public static DateTime ConvertFromUtc(DateTime utcDateTime, TimeZoneInfo targetTimeZone)
    {
        // Always validate the input is actually UTC
        if (utcDateTime.Kind != DateTimeKind.Utc)
        {
            throw new ArgumentException("Input must be UTC time");
        }

        return TimeZoneInfo.ConvertTimeFromUtc(utcDateTime, targetTimeZone);
    }

    public static DateTime ConvertToUtc(DateTime localDateTime, TimeZoneInfo sourceTimeZone)
    {
        return TimeZoneInfo.ConvertTimeToUtc(localDateTime, sourceTimeZone);
    }

    public static DateTime ConvertBetweenTimeZones(DateTime sourceTime, 
        TimeZoneInfo sourceTimeZone, TimeZoneInfo targetTimeZone)
    {
        // Convert to UTC first, then to target timezone
        var utcTime = TimeZoneInfo.ConvertTimeToUtc(sourceTime, sourceTimeZone);
        return TimeZoneInfo.ConvertTimeFromUtc(utcTime, targetTimeZone);
    }
}

// Usage examples
var utcNow = DateTime.UtcNow;
var easternTz = TimeZoneInfo.FindSystemTimeZoneById("Eastern Standard Time");
var pacificTz = TimeZoneInfo.FindSystemTimeZoneById("Pacific Standard Time");

var easternTime = TimeZoneConverter.ConvertFromUtc(utcNow, easternTz);
var pacificTime = TimeZoneConverter.ConvertFromUtc(utcNow, pacificTz);

Console.WriteLine($"UTC: {utcNow:yyyy-MM-dd HH:mm:ss}");
Console.WriteLine($"Eastern: {easternTime:yyyy-MM-dd HH:mm:ss}");
Console.WriteLine($"Pacific: {pacificTime:yyyy-MM-dd HH:mm:ss}");

Cross-Platform Timezone IDs

Windows and Linux use different timezone identifiers. Handle this gracefully :

public static class CrossPlatformTimeZones
{
    private static readonly Dictionary<string, string> TimeZoneMapping = new()
    {
        ["Eastern"] = RuntimeInformation.IsOSPlatform(OSPlatform.Windows) 
            ? "Eastern Standard Time" 
            : "America/New_York",
        ["Pacific"] = RuntimeInformation.IsOSPlatform(OSPlatform.Windows)
            ? "Pacific Standard Time"
            : "America/Los_Angeles", 
        ["Central"] = RuntimeInformation.IsOSPlatform(OSPlatform.Windows)
            ? "Central Standard Time"
            : "America/Chicago",
        ["Mountain"] = RuntimeInformation.IsOSPlatform(OSPlatform.Windows)
            ? "Mountain Standard Time"
            : "America/Denver"
    };

    public static TimeZoneInfo GetTimeZone(string commonName)
    {
        if (!TimeZoneMapping.TryGetValue(commonName, out var systemId))
        {
            throw new ArgumentException($"Unknown timezone: {commonName}");
        }

        try
        {
            return TimeZoneInfo.FindSystemTimeZoneById(systemId);
        }
        catch (TimeZoneNotFoundException)
        {
            throw new ArgumentException($"Timezone not found on this system: {systemId}");
        }
    }
}

Real-World Timezone Scenarios

User Profile with Timezone: Store user timezone preferences and convert timestamps for display :

public class UserService
{
    public class UserProfile
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public string TimeZoneId { get; set; }

        public TimeZoneInfo GetTimeZone()
        {
            try
            {
                return TimeZoneInfo.FindSystemTimeZoneById(TimeZoneId);
            }
            catch (TimeZoneNotFoundException)
            {
                // Fallback to UTC if user's timezone is not found
                return TimeZoneInfo.Utc;
            }
        }
    }

    public string FormatTimeForUser(DateTime utcDateTime, UserProfile user)
    {
        var userTimeZone = user.GetTimeZone();
        var localTime = TimeZoneInfo.ConvertTimeFromUtc(utcDateTime, userTimeZone);

        return localTime.ToString("yyyy-MM-dd HH:mm:ss") + 
               $" ({userTimeZone.GetUtcOffset(utcDateTime):hh\\:mm})";
    }
}

Meeting Scheduler: Handle meeting times across multiple timezones :

public class MeetingScheduler
{
    public class Meeting
    {
        public string Title { get; set; }
        public DateTime UtcStartTime { get; set; }  // Always store in UTC
        public TimeSpan Duration { get; set; }
        public List<int> AttendeeIds { get; set; }
    }

    public string GetMeetingTimeForUser(Meeting meeting, UserService.UserProfile user)
    {
        var userTimeZone = user.GetTimeZone();
        var localStartTime = TimeZoneInfo.ConvertTimeFromUtc(meeting.UtcStartTime, userTimeZone);
        var localEndTime = localStartTime.Add(meeting.Duration);

        return $"{meeting.Title}\n" +
               $"Start: {localStartTime:dddd, MMMM dd 'at' h:mm tt}\n" +
               $"End: {localEndTime:h:mm tt}\n" +
               $"Timezone: {userTimeZone.DisplayName}";
    }

    public List<string> GetMeetingTimesForAllAttendees(Meeting meeting, 
        List<UserService.UserProfile> attendees)
    {
        return attendees.Select(attendee => 
            $"{attendee.Name}: {GetMeetingTimeForUser(meeting, attendee)}")
            .ToList();
    }
}

Handling Timezone Edge Cases

Invalid Times: Some timezone transitions create invalid times :

public static class SafeTimeZoneConverter
{
    public static DateTime? ConvertToTimeZoneSafely(DateTime utcTime, TimeZoneInfo targetTimeZone)
    {
        try
        {
            var converted = TimeZoneInfo.ConvertTimeFromUtc(utcTime, targetTimeZone);

            // Check if the conversion resulted in an invalid time
            if (targetTimeZone.IsInvalidTime(converted))
            {
                Console.WriteLine($"Warning: {converted} is invalid in {targetTimeZone.Id}");
                return null;
            }

            return converted;
        }
        catch (ArgumentException ex)
        {
            Console.WriteLine($"Timezone conversion failed: {ex.Message}");
            return null;
        }
    }

    public static DateTime? ConvertFromTimeZoneSafely(DateTime localTime, TimeZoneInfo sourceTimeZone)
    {
        // Check for invalid or ambiguous times before conversion
        if (sourceTimeZone.IsInvalidTime(localTime))
        {
            Console.WriteLine($"Invalid time: {localTime} in {sourceTimeZone.Id}");
            return null;
        }

        if (sourceTimeZone.IsAmbiguousTime(localTime))
        {
            Console.WriteLine($"Ambiguous time: {localTime} in {sourceTimeZone.Id}");
            // Could ask user to clarify, or use a default assumption
        }

        try
        {
            return TimeZoneInfo.ConvertTimeToUtc(localTime, sourceTimeZone);
        }
        catch (ArgumentException ex)
        {
            Console.WriteLine($"Conversion failed: {ex.Message}");
            return null;
        }
    }
}

Common Timezone Mistakes

Storing local times in database: Always store UTC, convert for display :

// Bad - stores in user's timezone, causes confusion
public void SaveEventBad(Event eventData, UserService.UserProfile user)
{
    var userTimeZone = user.GetTimeZone();
    eventData.StartTime = TimeZoneInfo.ConvertTimeFromUtc(DateTime.UtcNow, userTimeZone);
    _db.Events.Add(eventData); // Stored in user's timezone - problematic!
}

// Good - always store UTC
public void SaveEventGood(Event eventData)
{
    eventData.StartTime = DateTime.UtcNow; // Always UTC in database
    _db.Events.Add(eventData);
}

Hardcoding timezone assumptions: Don't assume server timezone matches user needs :

// Bad - assumes server timezone is relevant to users
public string GetBusinessHours()
{
    var now = DateTime.Now; // Server timezone
    return now.Hour >= 9 && now.Hour < 17 ? "Open" : "Closed";
}

// Good - business hours relative to business location
public string GetBusinessHours(TimeZoneInfo businessTimeZone)
{
    var businessTime = TimeZoneInfo.ConvertTimeFromUtc(DateTime.UtcNow, businessTimeZone);
    return businessTime.Hour >= 9 && businessTime.Hour < 17 ? "Open" : "Closed";  
}

Chapter Summary

Use TimeZoneInfo for all timezone operations, always store UTC in databases, handle cross-platform timezone ID differences, and be aware of invalid/ambiguous times during DST transitions. Create utilities to safely convert between timezones.

Chapter Exercise: Build a WorldClockService that shows current time in multiple timezones. Include methods to add/remove timezones, handle timezone conversion errors gracefully, and format times appropriately for each locale. Test with DST transition periods.

---

Chapter 7: Dealing with Daylight Saving Time (DST)

Daylight Saving Time is perhaps the most frustrating aspect of date/time programming. Clocks "spring forward" and "fall back," creating hours that don't exist and hours that happen twice. Understanding DST behavior is crucial for building reliable applications that handle time transitions correctly.

Understanding DST Transitions

DST creates two problematic scenarios each year in affected timezones :

Spring Forward: Clocks jump from 2:00 AM to 3:00 AM, creating an "invalid" hour that never occurs.
Fall Back: Clocks repeat the hour from 1:00 AM to 2:00 AM, creating an "ambiguous" hour that occurs twice.

// Example: US Eastern timezone DST transitions in 2024
var easternTz = TimeZoneInfo.FindSystemTimeZoneById("Eastern Standard Time");

// Spring forward: March 10, 2024 at 2:00 AM EDT (invalid time)
var springForward = new DateTime(2024, 3, 10, 2, 30, 0); // This time never existed!
Console.WriteLine($"Is invalid: {easternTz.IsInvalidTime(springForward)}"); // True

// Fall back: November 3, 2024 at 1:30 AM (ambiguous time)  
var fallBack = new DateTime(2024, 11, 3, 1, 30, 0); // This time happened twice!
Console.WriteLine($"Is ambiguous: {easternTz.IsAmbiguousTime(fallBack)}"); // True

Detecting Invalid and Ambiguous Times

Always check for DST issues before performing timezone operations :

public static class DstChecker
{
    public static void AnalyzeDateTime(DateTime dateTime, TimeZoneInfo timeZone)
    {
        Console.WriteLine($"Analyzing: {dateTime:yyyy-MM-dd HH:mm:ss} in {timeZone.Id}");

        if (timeZone.IsInvalidTime(dateTime))
        {
            Console.WriteLine("⚠️  INVALID TIME - This time never existed due to DST spring forward");

            // Show what the adjustment rules are
            var adjustmentRules = timeZone.GetAdjustmentRules();
            var applicableRule = adjustmentRules.FirstOrDefault(rule => 
                dateTime.Date >= rule.DateStart && dateTime.Date <= rule.DateEnd);

            if (applicableRule != null)
            {
                Console.WriteLine($"DST starts: {applicableRule.DaylightTransitionStart}");
                Console.WriteLine($"DST ends: {applicableRule.DaylightTransitionEnd}");
            }
        }
        else if (timeZone.IsAmbiguousTime(dateTime))
        {
            Console.WriteLine("⚠️  AMBIGUOUS TIME - This time occurred twice due to DST fall back");

            // Show both possible UTC times
            var possibleUtcTimes = timeZone.GetAmbiguousTimeOffsets(dateTime);
            Console.WriteLine("Possible interpretations:");

            foreach (var offset in possibleUtcTimes)
            {
                var utcTime = dateTime - offset;
                Console.WriteLine($"  UTC: {utcTime:yyyy-MM-dd HH:mm:ss} (offset: {offset})");
            }
        }
        else
        {
            Console.WriteLine("✅ Valid, unambiguous time");
            var utcOffset = timeZone.GetUtcOffset(dateTime);
            Console.WriteLine($"UTC offset: {utcOffset}");
        }
    }
}

Safe DST Conversions

Handle DST transitions safely with proper error checking :

public static class SafeDstConverter
{
    public static DateTime? ConvertToUtcSafely(DateTime localTime, TimeZoneInfo sourceTimeZone)
    {
        // Check for invalid time (spring forward)
        if (sourceTimeZone.IsInvalidTime(localTime))
        {
            Console.WriteLine($"Cannot convert invalid time: {localTime}");
            return null;
        }

        // Handle ambiguous time (fall back)
        if (sourceTimeZone.IsAmbiguousTime(localTime))
        {
            Console.WriteLine($"Ambiguous time detected: {localTime}");

            // Default to standard time interpretation (first occurrence)
            var offsets = sourceTimeZone.GetAmbiguousTimeOffsets(localTime);
            var standardOffset = offsets.Max(); // Standard time has larger offset

            return localTime - standardOffset;
        }

        // Normal conversion
        return TimeZoneInfo.ConvertTimeToUtc(localTime, sourceTimeZone);
    }

    public static DateTime? ConvertFromUtcSafely(DateTime utcTime, TimeZoneInfo targetTimeZone)
    {
        if (utcTime.Kind != DateTimeKind.Utc)
        {
            throw new ArgumentException("Input must be UTC");
        }

        try
        {
            var converted = TimeZoneInfo.ConvertTimeFromUtc(utcTime, targetTimeZone);

            // Double-check the result isn't invalid (shouldn't happen from UTC)
            if (targetTimeZone.IsInvalidTime(converted))
            {
                Console.WriteLine($"Unexpected invalid result: {converted}");
                return null;
            }

            return converted;
        }
        catch (ArgumentException ex)
        {
            Console.WriteLine($"Conversion failed: {ex.Message}");
            return null;
        }
    }
}

Business Logic During DST

Different business scenarios need different DST handling strategies :

public class DstBusinessLogic
{
    // Example 1: Daily recurring task at 2:30 AM
    public static DateTime? GetNextDailyTaskTime(DateTime currentUtc, TimeZoneInfo businessTimeZone)
    {
        var businessTime = TimeZoneInfo.ConvertTimeFromUtc(currentUtc, businessTimeZone);
        var nextDay = businessTime.Date.AddDays(1);
        var targetTime = nextDay.AddHours(2).AddMinutes(30); // 2:30 AM

        // Check if target time is invalid (spring forward)
        if (businessTimeZone.IsInvalidTime(targetTime))
        {
            Console.WriteLine("Target time falls in DST gap, adjusting to 3:30 AM");
            targetTime = targetTime.AddHours(1); // Move to 3:30 AM
        }

        // Convert back to UTC for storage
        return SafeDstConverter.ConvertToUtcSafely(targetTime, businessTimeZone);
    }

    // Example 2: Business hours calculation spanning DST
    public static TimeSpan CalculateBusinessHours(DateTime startUtc, DateTime endUtc, 
        TimeZoneInfo businessTimeZone)
    {
        var startLocal = TimeZoneInfo.ConvertTimeFromUtc(startUtc, businessTimeZone);
        var endLocal = TimeZoneInfo.ConvertTimeFromUtc(endUtc, businessTimeZone);

        var totalHours = TimeSpan.Zero;
        var current = startLocal.Date;

        while (current <= endLocal.Date)
        {
            var dayStart = current.AddHours(9); // 9 AM
            var dayEnd = current.AddHours(17);   // 5 PM

            // Adjust for actual start/end times
            if (current == startLocal.Date && startLocal > dayStart)
                dayStart = startLocal;
            if (current == endLocal.Date && endLocal < dayEnd)
                dayEnd = endLocal;

            // Skip weekends
            if (current.DayOfWeek != DayOfWeek.Saturday && 
                current.DayOfWeek != DayOfWeek.Sunday && 
                dayEnd > dayStart)
            {
                // Handle DST transitions within business hours
                var businessHoursThisDay = CalculateDayHours(dayStart, dayEnd, businessTimeZone);
                totalHours = totalHours.Add(businessHoursThisDay);
            }

            current = current.AddDays(1);
        }

        return totalHours;
    }

    private static TimeSpan CalculateDayHours(DateTime start, DateTime end, TimeZoneInfo timeZone)
    {
        // Convert to UTC to get accurate duration (handles DST automatically)
        var startUtc = SafeDstConverter.ConvertToUtcSafely(start, timeZone) ?? start;
        var endUtc = SafeDstConverter.ConvertToUtcSafely(end, timeZone) ?? end;

        return endUtc - startUtc;
    }
}

Testing DST Scenarios

Create comprehensive tests for DST edge cases :

[TestFixture]
public class DstTests
{
    private TimeZoneInfo _easternTz;

    [SetUp]
    public void Setup()
    {
        _easternTz = TimeZoneInfo.FindSystemTimeZoneById("Eastern Standard Time");
    }

    [Test]
    public void SpringForward_InvalidTime_HandledCorrectly()
    {
        // March 10, 2024 - spring forward at 2:00 AM
        var invalidTime = new DateTime(2024, 3, 10, 2, 30, 0);

        Assert.IsTrue(_easternTz.IsInvalidTime(invalidTime));

        var result = SafeDstConverter.ConvertToUtcSafely(invalidTime, _easternTz);
        Assert.IsNull(result, "Invalid time should return null");
    }

    [Test]
    public void FallBack_AmbiguousTime_DefaultsToStandardTime()
    {
        // November 3, 2024 - fall back at 2:00 AM
        var ambiguousTime = new DateTime(2024, 11, 3, 1, 30, 0);

        Assert.IsTrue(_easternTz.IsAmbiguousTime(ambiguousTime));

        var result = SafeDstConverter.ConvertToUtcSafely(ambiguousTime, _easternTz);
        Assert.IsNotNull(result);

        // Should interpret as standard time (first occurrence after fall back)
        var expectedUtc = new DateTime(2024, 11, 3, 6, 30, 0, DateTimeKind.Utc);
        Assert.AreEqual(expectedUtc, result);
    }

    [Test]
    public void RecurringTask_SkipsInvalidTime()
    {
        // Test daily task that normally runs at 2:30 AM
        var beforeSpringForward = new DateTime(2024, 3, 9, 12, 0, 0, DateTimeKind.Utc);

        var nextTask = DstBusinessLogic.GetNextDailyTaskTime(beforeSpringForward, _easternTz);
        Assert.IsNotNull(nextTask);

        var nextTaskLocal = TimeZoneInfo.ConvertTimeFromUtc(nextTask.Value, _easternTz);

        // Should be 3:30 AM due to DST adjustment
        Assert.AreEqual(3, nextTaskLocal.Hour);
        Assert.AreEqual(30, nextTaskLocal.Minute);
    }
}

DST-Aware Logging

Log timestamps in a way that makes DST transitions clear :

public static class DstAwareLogger
{
    public static void LogWithDstInfo(string message, TimeZoneInfo logTimeZone = null)
    {
        var utcNow = DateTime.UtcNow;
        logTimeZone ??= TimeZoneInfo.Local;

        var localTime = TimeZoneInfo.ConvertTimeFromUtc(utcNow, logTimeZone);
        var offset = logTimeZone.GetUtcOffset(utcNow);
        var isDst = logTimeZone.IsDaylightSavingTime(localTime);

        var timestampInfo = $"{localTime:yyyy-MM-dd HH:mm:ss.fff} " +
                           $"({offset:hh\\:mm}) " +
                           $"{(isDst ? "DST" : "STD")} " +
                           $"[UTC: {utcNow:yyyy-MM-dd HH:mm:ss.fff}]";

        Console.WriteLine($"{timestampInfo} - {message}");
    }
}

// Usage during DST transition
DstAwareLogger.LogWithDstInfo("Application started");
// Output: 2024-03-10 03:15:30.123 (-04:00) DST [UTC: 2024-03-10 07:15:30.123] - Application started

Chapter Summary

DST creates invalid times (spring forward) and ambiguous times (fall back) that must be handled explicitly. Always check for DST issues before timezone conversions, provide sensible defaults for ambiguous times, and test thoroughly around DST transition dates. Store times in UTC to avoid most DST complications.

Chapter Exercise: Build a DstSafeScheduler class that can schedule recurring events (daily, weekly) while properly handling DST transitions. Include methods to detect when scheduled times fall into invalid periods and provide alternative times. Test with both spring forward and fall back scenarios.

---

Chapter 8: Measuring Time and Durations

Ever wondered why your "quick" API call occasionally takes 10 seconds instead of 100 milliseconds? Or needed to calculate how many business hours elapsed between two dates? Measuring time accurately is crucial for performance monitoring, SLA tracking, billing calculations, and understanding user behavior. But here's the catch - not all timing tools are created equal, and using the wrong one can give you misleading results.[1]

In performance-critical applications, microseconds matter. In billing systems, accurate duration calculations directly impact revenue. Let's master the tools .NET provides for measuring time and durations so you can choose the right approach for each scenario.

Understanding TimeSpan for Durations

TimeSpan represents a duration or interval between two points in time - it's not a point in time itself. Think of it as "how long" rather than "when".[1]

// Creating TimeSpan in various ways
var oneHour = TimeSpan.FromHours(1);
var thirtyMinutes = TimeSpan.FromMinutes(30);
var fiveSeconds = TimeSpan.FromSeconds(5);
var twoMilliseconds = TimeSpan.FromMilliseconds(2);

// Constructor approach: days, hours, minutes, seconds, milliseconds
var specificDuration = new TimeSpan(1, 2, 30, 45, 500); // 1 day, 2 hours, 30 minutes, 45 seconds, 500ms

// From ticks (most precise)
var fromTicks = new TimeSpan(10_000_000); // 1 second = 10,000,000 ticks

Console.WriteLine($"One hour: {oneHour}"); // 01:00:00
Console.WriteLine($"Specific: {specificDuration}"); // 1.02:30:45.5000000
Console.WriteLine($"Total minutes: {oneHour.TotalMinutes}"); // 60

Calculating Time Differences

TimeSpan naturally results from subtracting DateTime values - this is perfect for measuring elapsed time :[1]

public static class TimeDifferenceCalculator
{
    public static void CalculateEventDuration()
    {
        var eventStart = new DateTime(2024, 3, 15, 9, 0, 0, DateTimeKind.Utc);
        var eventEnd = new DateTime(2024, 3, 15, 17, 30, 0, DateTimeKind.Utc);

        var duration = eventEnd - eventStart;

        Console.WriteLine($"Event duration: {duration}"); // 08:30:00
        Console.WriteLine($"Total hours: {duration.TotalHours}"); // 8.5
        Console.WriteLine($"Total minutes: {duration.TotalMinutes}"); // 510
    }

    public static void CalculateAge()
    {
        var birthDate = new DateTime(1990, 6, 15);
        var today = DateTime.Today;

        var age = today - birthDate;
        var years = (int)(age.TotalDays / 365.25); // Approximate

        Console.WriteLine($"Age: {years} years");
        Console.WriteLine($"Exact days: {age.TotalDays:F0} days");
    }
}

Stopwatch for High-Precision Timing

When you need accurate performance measurements, Stopwatch is your best friend. It uses the highest-resolution timer available on the system :[1]

public static class PerformanceTiming
{
    public static void MeasureOperationPerformance()
    {
        var stopwatch = Stopwatch.StartNew();

        // Perform some operation
        SimulateWork();

        stopwatch.Stop();

        Console.WriteLine($"Elapsed time: {stopwatch.Elapsed}"); // TimeSpan
        Console.WriteLine($"Elapsed milliseconds: {stopwatch.ElapsedMilliseconds}"); // long
        Console.WriteLine($"Elapsed ticks: {stopwatch.ElapsedTicks}"); // long (highest precision)
    }

    public static void CompareOperations()
    {
        var method1Time = MeasureMethod(() => SlowMethod());
        var method2Time = MeasureMethod(() => FastMethod());

        Console.WriteLine($"Slow method: {method1Time.TotalMilliseconds:F2}ms");
        Console.WriteLine($"Fast method: {method2Time.TotalMilliseconds:F2}ms");
        Console.WriteLine($"Speed improvement: {method1Time.TotalMilliseconds / method2Time.TotalMilliseconds:F1}x");
    }

    private static TimeSpan MeasureMethod(Action method)
    {
        var sw = Stopwatch.StartNew();
        method();
        sw.Stop();
        return sw.Elapsed;
    }

    private static void SimulateWork() => Thread.Sleep(100);
    private static void SlowMethod() => Thread.Sleep(50);
    private static void FastMethod() => Thread.Sleep(10);
}

Performance Comparison: DateTime vs Stopwatch

Here's an important distinction - DateTime arithmetic vs Stopwatch for measuring elapsed time :[1]

// ❌ Bad: Using DateTime for performance measurement
var start = DateTime.UtcNow;
PerformSomeOperation();
var end = DateTime.UtcNow;
var elapsed = end - start; // Less accurate, affected by system clock adjustments

// ✅ Good: Using Stopwatch for performance measurement
var stopwatch = Stopwatch.StartNew();
PerformSomeOperation();
stopwatch.Stop();
var elapsed = stopwatch.Elapsed; // High precision, immune to clock adjustments

Why Stopwatch is better for performance measurement:

• Uses high-resolution performance counters when available
• Not affected by system clock adjustments (NTP sync, DST changes)
• Designed specifically for measuring short durations accurately

Real-World Duration Scenarios

Let's look at practical applications where accurate time measurement matters:

public class BillingCalculator
{
    public decimal CalculateUsageBilling(DateTime startUtc, DateTime endUtc, decimal hourlyRate)
    {
        // Always use UTC for billing to avoid timezone confusion
        if (startUtc.Kind != DateTimeKind.Utc || endUtc.Kind != DateTimeKind.Utc)
            throw new ArgumentException("Billing times must be in UTC");

        var duration = endUtc - startUtc;
        var billableHours = (decimal)duration.TotalHours;

        // Round up to nearest minute for billing
        var billableMinutes = Math.Ceiling(duration.TotalMinutes);
        var adjustedHours = billableMinutes / 60;

        return adjustedHours * hourlyRate;
    }
}

public class SlaMonitor
{
    private readonly Dictionary<string, Stopwatch> _operationTimers = new();

    public void StartOperation(string operationId)
    {
        _operationTimers[operationId] = Stopwatch.StartNew();
    }

    public TimeSpan? CompleteOperation(string operationId)
    {
        if (!_operationTimers.TryGetValue(operationId, out var timer))
            return null;

        timer.Stop();
        var elapsed = timer.Elapsed;
        _operationTimers.Remove(operationId);

        // Check SLA compliance (e.g., must complete within 5 seconds)
        if (elapsed.TotalSeconds > 5)
        {
            LogSlaBreach(operationId, elapsed);
        }

        return elapsed;
    }

    private void LogSlaBreach(string operationId, TimeSpan elapsed)
    {
        Console.WriteLine($"SLA BREACH: {operationId} took {elapsed.TotalSeconds:F2}s");
    }
}

Working with Business Hours and Complex Durations

Sometimes you need to calculate durations that exclude weekends or non-business hours:

public static class BusinessDurationCalculator
{
    public static TimeSpan CalculateBusinessHours(DateTime startUtc, DateTime endUtc, TimeZoneInfo businessTimeZone)
    {
        var startLocal = TimeZoneInfo.ConvertTimeFromUtc(startUtc, businessTimeZone);
        var endLocal = TimeZoneInfo.ConvertTimeFromUtc(endUtc, businessTimeZone);

        var totalBusinessHours = TimeSpan.Zero;
        var current = startLocal.Date;

        while (current <= endLocal.Date)
        {
            // Skip weekends
            if (current.DayOfWeek == DayOfWeek.Saturday || current.DayOfWeek == DayOfWeek.Sunday)
            {
                current = current.AddDays(1);
                continue;
            }

            // Business hours: 9 AM to 5 PM
            var dayStart = current.AddHours(9);
            var dayEnd = current.AddHours(17);

            // Adjust for actual start/end times
            if (current == startLocal.Date && startLocal > dayStart)
                dayStart = startLocal;
            if (current == endLocal.Date && endLocal < dayEnd)
                dayEnd = endLocal;

            // Only count if we have valid business hours this day
            if (dayEnd > dayStart)
                totalBusinessHours = totalBusinessHours.Add(dayEnd - dayStart);

            current = current.AddDays(1);
        }

        return totalBusinessHours;
    }
}

Common Duration Mistakes

Using DateTime.Now for performance measurement:

// ❌ Bad: Affected by system clock changes
var start = DateTime.Now;
DoWork();
var elapsed = DateTime.Now - start;

// ✅ Good: High-precision, immune to clock changes
var sw = Stopwatch.StartNew();
DoWork();
var elapsed = sw.Elapsed;

Forgetting timezone context in duration calculations:

// ❌ Problematic: What timezone are these in?
public TimeSpan CalculateWorkDuration(DateTime start, DateTime end)
{
    return end - start; // Could be wrong if times are in different timezones
}

// ✅ Better: Always work in UTC for duration calculations
public TimeSpan CalculateWorkDuration(DateTime startUtc, DateTime endUtc)
{
    if (startUtc.Kind != DateTimeKind.Utc || endUtc.Kind != DateTimeKind.Utc)
        throw new ArgumentException("Times must be in UTC for accurate duration calculation");

    return endUtc - startUtc;
}

Chapter Summary

Use TimeSpan for representing durations and intervals between DateTime values. Use Stopwatch for high-precision performance measurement - it's more accurate than DateTime arithmetic for timing operations. Always consider timezone context when calculating durations, and prefer UTC for accurate duration calculations. Create specialized calculators for complex scenarios like business hours or billing periods.

Chapter Exercise

Build a PerformanceProfiler class that can:

1. Measure and compare the execution time of different methods
2. Calculate percentile statistics (median, 95th percentile) over multiple runs
3. Format timing results in human-readable format (ms, seconds, etc.)
4. Detect performance regressions by comparing against baseline measurements

Test it by profiling different sorting algorithms with various data sizes.

---

Chapter 9: Working with DateOnly and TimeOnly in .NET 6+

Picture this: you're building a birthday reminder system and keep getting confused by timezone-related bugs. Or you're creating a recurring schedule system where you only care about "3:30 PM" regardless of the date. The classic DateTime struct tries to be everything to everyone, but sometimes you just need a date OR a time, not both.[1]

.NET 6 introduced two specialized types that solve these exact problems: DateOnly for when you only care about dates, and TimeOnly for when you only care about times. These types eliminate timezone confusion and make your intent crystal clear.

Understanding DateOnly

DateOnly represents just a date - no time component, no timezone complications. It's perfect for scenarios like birthdays, holidays, event dates, and any business logic that deals purely with calendar dates [1].

// Creating DateOnly instances
var today = DateOnly.FromDateTime(DateTime.Today);
var specificDate = new DateOnly(2024, 3, 15);        // March 15, 2024
var christmas = new DateOnly(2024, 12, 25);

// Common operations
var dayOfWeek = specificDate.DayOfWeek;               // Friday
var dayOfYear = specificDate.DayOfYear;               // 75
var isLeapYear = DateOnly.IsLeapYear(2024);           // true

Console.WriteLine($"Today: {today}");                 // 2024-09-29
Console.WriteLine($"Christmas: {christmas}");         // 2024-12-25
Console.WriteLine($"Days until Christmas: {christmas.DayNumber - today.DayNumber}");

Real-world DateOnly scenarios include user profile birthdates, holiday calendars, project deadlines, and subscription renewal dates. Since DateOnly has no timezone component, you never have to worry about "is this user's birthday in their timezone or UTC?"

Understanding TimeOnly

TimeOnly represents just a time of day - no date, no timezone. It's perfect for recurring schedules, business hours, daily reminders, and any logic that repeats daily [1].

// Creating TimeOnly instances  
var lunchTime = new TimeOnly(12, 30);                // 12:30 PM
var closingTime = TimeOnly.FromDateTime(DateTime.Now); // Extract time from DateTime
var precise = new TimeOnly(14, 15, 30, 500);         // 2:15:30.500 PM

// Common operations
var totalMinutes = lunchTime.ToTimeSpan().TotalMinutes; // 750 minutes since midnight
var formatted = lunchTime.ToString("hh:mm tt");         // 12:30 PM
var is24Hour = new TimeOnly(23, 59);                    // 11:59 PM

Console.WriteLine($"Lunch time: {lunchTime}");          // 12:30:00
Console.WriteLine($"In 12-hour format: {formatted}");   // 12:30 PM

// Time comparisons
if (TimeOnly.FromDateTime(DateTime.Now) > lunchTime)
{
    Console.WriteLine("Lunch time has passed");
}

Converting Between DateTime and DateOnly/TimeOnly

The new types work seamlessly with existing DateTime code through explicit conversion methods:

public static class DateTimeConversions
{
    public static void DemonstrateConversions()
    {
        var now = DateTime.Now;

        // Extract date and time components
        var dateOnly = DateOnly.FromDateTime(now);
        var timeOnly = TimeOnly.FromDateTime(now);

        Console.WriteLine($"Original: {now}");
        Console.WriteLine($"Date part: {dateOnly}");
        Console.WriteLine($"Time part: {timeOnly}");

        // Combine them back (assumes Kind.Unspecified)
        var combined = dateOnly.ToDateTime(timeOnly);
        Console.WriteLine($"Combined: {combined}");

        // For UTC times, specify the kind
        var utcCombined = dateOnly.ToDateTime(timeOnly, DateTimeKind.Utc);
        Console.WriteLine($"UTC Combined: {utcCombined}");
    }

    public static DateTime CreateUtcDateTime(DateOnly date, TimeOnly time)
    {
        return date.ToDateTime(time, DateTimeKind.Utc);
    }
}

Practical Applications with DateOnly and TimeOnly

Let's see how these types solve real problems:

// Birthday system - no timezone confusion
public class BirthdayManager
{
    public class UserProfile
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public DateOnly Birthday { get; set; } // Just a date, no time/timezone issues
    }

    public List<UserProfile> GetBirthdaysToday()
    {
        var today = DateOnly.FromDateTime(DateTime.Today);
        return users.Where(user => 
            user.Birthday.Month == today.Month && 
            user.Birthday.Day == today.Day
        ).ToList();
    }

    public int CalculateAge(DateOnly birthday)
    {
        var today = DateOnly.FromDateTime(DateTime.Today);
        var age = today.Year - birthday.Year;

        // Adjust if birthday hasn't occurred this year
        if (today.Month < birthday.Month || 
            (today.Month == birthday.Month && today.Day < birthday.Day))
        {
            age--;
        }

        return age;
    }
}

// Business hours system - time without dates
public class BusinessHours
{
    public TimeOnly OpenTime { get; set; }
    public TimeOnly CloseTime { get; set; }

    public bool IsOpen(TimeOnly currentTime)
    {
        // Handle cases where business is open past midnight
        if (CloseTime < OpenTime) // e.g., 22:00 to 06:00
        {
            return currentTime >= OpenTime || currentTime <= CloseTime;
        }

        return currentTime >= OpenTime && currentTime <= CloseTime;
    }

    public string GetStatus()
    {
        var now = TimeOnly.FromDateTime(DateTime.Now);
        return IsOpen(now) ? "Open" : "Closed";
    }
}

// Recurring schedule system
public class RecurringSchedule
{
    public class DailyTask
    {
        public string Name { get; set; }
        public TimeOnly ScheduledTime { get; set; }
        public List<DayOfWeek> ActiveDays { get; set; } = new();
    }

    public List<DailyTask> GetTasksForToday()
    {
        var today = DateTime.Today.DayOfWeek;
        var currentTime = TimeOnly.FromDateTime(DateTime.Now);

        return dailyTasks.Where(task => 
            task.ActiveDays.Contains(today) && 
            task.ScheduledTime > currentTime // Future tasks today
        ).OrderBy(task => task.ScheduledTime).ToList();
    }

    public DateTime GetNextOccurrence(DailyTask task)
    {
        var today = DateOnly.FromDateTime(DateTime.Today);
        var todayOfWeek = DateTime.Today.DayOfWeek;

        // If task runs today and time hasn't passed
        if (task.ActiveDays.Contains(todayOfWeek))
        {
            var todayDateTime = today.ToDateTime(task.ScheduledTime);
            if (todayDateTime > DateTime.Now)
            {
                return todayDateTime;
            }
        }

        // Find next active day
        for (int i = 1; i <= 7; i++)
        {
            var futureDate = today.AddDays(i);
            var futureDayOfWeek = futureDate.ToDateTime(TimeOnly.MinValue).DayOfWeek;

            if (task.ActiveDays.Contains(futureDayOfWeek))
            {
                return futureDate.ToDateTime(task.ScheduledTime);
            }
        }

        throw new InvalidOperationException("Task has no active days");
    }
}

Database Storage with DateOnly and TimeOnly

Entity Framework Core 6+ has built-in support for these types:

public class AppDbContext : DbContext
{
    public DbSet<Employee> Employees { get; set; }
    public DbSet<WorkSchedule> WorkSchedules { get; set; }
}

public class Employee
{
    public int Id { get; set; }
    public string Name { get; set; }
    public DateOnly HireDate { get; set; }      // Stored as DATE in SQL
    public DateOnly? Birthday { get; set; }     // Nullable date
}

public class WorkSchedule
{
    public int Id { get; set; }
    public string ShiftName { get; set; }
    public TimeOnly StartTime { get; set; }     // Stored as TIME in SQL
    public TimeOnly EndTime { get; set; }
    public List<DayOfWeek> WorkDays { get; set; } // JSON or separate table
}

Performance and Memory Considerations

DateOnly and TimeOnly are more memory-efficient than DateTime for their specific use cases:

public static class MemoryComparison
{
    public static void ShowMemoryUsage()
    {
        // DateTime: 8 bytes + Kind info
        var dateTime = DateTime.Now;

        // DateOnly: 4 bytes (day number since year 1)
        var dateOnly = DateOnly.FromDateTime(dateTime);

        // TimeOnly: 8 bytes (ticks since midnight)
        var timeOnly = TimeOnly.FromDateTime(dateTime);

        Console.WriteLine($"DateTime size: ~8 bytes");
        Console.WriteLine($"DateOnly size: 4 bytes");
        Console.WriteLine($"TimeOnly size: 8 bytes");
        Console.WriteLine($"Combined DateOnly + TimeOnly: 12 bytes vs DateTime: 8 bytes");
        // Trade-off: Slightly more memory for much clearer semantics
    }
}

Migration Strategies

If you have existing DateTime properties that only use date or time, here's how to migrate:

// Before: Using DateTime for date-only data
public class EventOld
{
    public DateTime EventDate { get; set; } // Time portion ignored, confusing
}

// After: Using DateOnly for clarity
public class EventNew
{
    public DateOnly EventDate { get; set; } // Clear intent: just a date
}

// Migration helper
public static class DateTimeMigration
{
    public static DateOnly ExtractDateSafely(DateTime dateTime)
    {
        // Always use the date portion, ignore time/timezone
        return DateOnly.FromDateTime(dateTime.Date);
    }

    public static TimeOnly ExtractTimeSafely(DateTime dateTime)
    {
        // Extract time portion, ignore date
        return TimeOnly.FromDateTime(dateTime);
    }
}

Common Patterns and Best Practices

Use DateOnly for:

• User birthdays and anniversaries
• Holiday calendars and recurring events
• Project deadlines and milestones
• Date-based business rules (fiscal years, etc.)

Use TimeOnly for:

• Daily schedules and recurring times
• Business hours and operating schedules
• Time-based triggers (daily backups at 2 AM)
• Duration-independent time calculations

Avoid these types when:

• You need timezone information (stick with DateTime/DateTimeOffset)
• You're working with precise timestamps (logging, auditing)
• Interfacing with legacy systems that expect DateTime

Chapter Summary

DateOnly and TimeOnly provide clear, specialized types for date-only and time-only scenarios. They eliminate timezone confusion in scenarios where timezone isn't relevant, provide better semantic clarity than DateTime, and integrate well with modern .NET features like Entity Framework Core. Use them to make your intent explicit and reduce timezone-related bugs.

Chapter Exercise

Build a CalendarService class that manages events and recurring schedules:

1. Use DateOnly for event dates and TimeOnly for event times
2. Support recurring events (daily, weekly, monthly)
3. Handle events that span midnight (using TimeOnly)
4. Provide methods to find all events for a given date range
5. Include timezone conversion utilities for displaying events to users in different timezones

Test with various recurring patterns and edge cases like leap years and daylight saving time transitions.

Chapter 10: Using TimeProvider in .NET 8+

Testing time-dependent code has always been a nightmare. How do you test a method that behaves differently depending on the current time? How do you verify that your scheduled job runs at the right time without waiting hours? Mocking DateTime.Now requires complex workarounds that make tests brittle and hard to maintain.[1]

.NET 8 introduced TimeProvider - a new abstraction that solves these problems elegantly. It makes time testable while keeping your production code simple and performant. Let's explore how this game-changing feature works.

Understanding TimeProvider Abstraction

TimeProvider is an abstract class that provides time-related services. The framework includes SystemTimeProvider for production use, and you can create custom providers for testing [1]:

// The TimeProvider abstraction
public abstract class TimeProvider
{
    public static TimeProvider System { get; } // Default system time provider

    public abstract DateTimeOffset GetUtcNow();
    public abstract TimeZoneInfo LocalTimeZone { get; }

    public virtual DateTimeOffset GetLocalNow() => TimeZoneInfo.ConvertTime(GetUtcNow(), LocalTimeZone);

    public virtual ITimer CreateTimer(TimerCallback callback, object? state, TimeSpan dueTime, TimeSpan period);

    // Other methods for time-based operations
}

// Usage in production code
public class OrderService
{
    private readonly TimeProvider _timeProvider;

    public OrderService(TimeProvider timeProvider)
    {
        _timeProvider = timeProvider ?? TimeProvider.System;
    }

    public Order CreateOrder(Customer customer)
    {
        return new Order
        {
            CustomerId = customer.Id,
            CreatedAt = _timeProvider.GetUtcNow().DateTime, // Always UTC
            Status = OrderStatus.Pending
        };
    }

    public bool IsOrderExpired(Order order, TimeSpan expirationPeriod)
    {
        var now = _timeProvider.GetUtcNow();
        var expirationTime = order.CreatedAt.Add(expirationPeriod);
        return now.DateTime > expirationTime;
    }
}

Creating Testable Time-Dependent Code

With TimeProvider, you can write clean, testable code that doesn't depend on the system clock:

public class SubscriptionService
{
    private readonly TimeProvider _timeProvider;
    private readonly ILogger<SubscriptionService> _logger;

    public SubscriptionService(TimeProvider timeProvider, ILogger<SubscriptionService> logger)
    {
        _timeProvider = timeProvider;
        _logger = logger;
    }

    public class Subscription
    {
        public int Id { get; set; }
        public DateTime StartDate { get; set; }
        public DateTime EndDate { get; set; }
        public bool IsActive { get; set; }
    }

    public Subscription CreateAnnualSubscription(int userId)
    {
        var now = _timeProvider.GetUtcNow().DateTime;
        var subscription = new Subscription
        {
            Id = userId,
            StartDate = now,
            EndDate = now.AddYears(1),
            IsActive = true
        };

        _logger.LogInformation("Created subscription {SubscriptionId} valid until {EndDate}", 
            subscription.Id, subscription.EndDate);

        return subscription;
    }

    public List<Subscription> GetExpiredSubscriptions(List<Subscription> subscriptions)
    {
        var now = _timeProvider.GetUtcNow().DateTime;
        return subscriptions.Where(s => s.EndDate < now).ToList();
    }

    public TimeSpan GetTimeUntilExpiration(Subscription subscription)
    {
        var now = _timeProvider.GetUtcNow().DateTime;
        return subscription.EndDate > now ? subscription.EndDate - now : TimeSpan.Zero;
    }
}

Dependency Injection with TimeProvider

Register TimeProvider in your DI container for clean separation of concerns:

// Program.cs or Startup.cs
public static class ServiceConfiguration
{
    public static IServiceCollection AddTimeServices(this IServiceCollection services)
    {
        // Use system time in production
        services.AddSingleton<TimeProvider>(TimeProvider.System);

        // Register time-dependent services
        services.AddScoped<OrderService>();
        services.AddScoped<SubscriptionService>();
        services.AddScoped<AuditLogger>();

        return services;
    }
}

// Usage in controllers or services
[ApiController]
[Route("[controller]")]
public class OrdersController : ControllerBase
{
    private readonly OrderService _orderService;

    public OrdersController(OrderService orderService)
    {
        _orderService = orderService;
    }

    [HttpPost]
    public IActionResult CreateOrder(CreateOrderRequest request)
    {
        var customer = new Customer { Id = request.CustomerId };
        var order = _orderService.CreateOrder(customer);

        return CreatedAtAction(nameof(GetOrder), new { id = order.Id }, order);
    }
}

Custom TimeProvider for Testing

Create a fake TimeProvider that lets you control time in tests:

public class FakeTimeProvider : TimeProvider
{
    private DateTimeOffset _currentTime;

    public FakeTimeProvider(DateTimeOffset startTime)
    {
        _currentTime = startTime;
    }

    public override DateTimeOffset GetUtcNow() => _currentTime;

    public override TimeZoneInfo LocalTimeZone => TimeZoneInfo.Utc; // Simplified for testing

    // Methods to control time in tests
    public void SetTime(DateTimeOffset time) => _currentTime = time;

    public void AdvanceTime(TimeSpan duration) => _currentTime = _currentTime.Add(duration);

    public void AdvanceTimeBy(int days = 0, int hours = 0, int minutes = 0, int seconds = 0)
    {
        _currentTime = _currentTime.AddDays(days).AddHours(hours).AddMinutes(minutes).AddSeconds(seconds);
    }

    // Simplified timer implementation for testing
    public override ITimer CreateTimer(TimerCallback callback, object? state, TimeSpan dueTime, TimeSpan period)
    {
        return new FakeTimer(callback, state, dueTime, period, this);
    }
}

// Simple fake timer for testing
public class FakeTimer : ITimer
{
    private readonly TimerCallback _callback;
    private readonly object? _state;
    private bool _disposed;

    public FakeTimer(TimerCallback callback, object? state, TimeSpan dueTime, TimeSpan period, FakeTimeProvider timeProvider)
    {
        _callback = callback;
        _state = state;
        // In a real implementation, you'd integrate with the fake time provider
        // For simplicity, we'll just store the values
    }

    public bool Change(TimeSpan dueTime, TimeSpan period) => !_disposed;

    public void Dispose() => _disposed = true;

    public ValueTask DisposeAsync()
    {
        Dispose();
        return ValueTask.CompletedTask;
    }
}

Comprehensive Unit Testing with TimeProvider

Now testing time-dependent code becomes straightforward:

[TestFixture]
public class SubscriptionServiceTests
{
    private FakeTimeProvider _timeProvider;
    private SubscriptionService _subscriptionService;
    private ILogger<SubscriptionService> _logger;

    [SetUp]
    public void Setup()
    {
        _timeProvider = new FakeTimeProvider(new DateTimeOffset(2024, 3, 15, 12, 0, 0, TimeSpan.Zero));
        _logger = Substitute.For<ILogger<SubscriptionService>>();
        _subscriptionService = new SubscriptionService(_timeProvider, _logger);
    }

    [Test]
    public void CreateAnnualSubscription_SetsCorrectDates()
    {
        // Arrange
        var userId = 123;
        var expectedStartDate = new DateTime(2024, 3, 15, 12, 0, 0);
        var expectedEndDate = expectedStartDate.AddYears(1);

        // Act
        var subscription = _subscriptionService.CreateAnnualSubscription(userId);

        // Assert
        Assert.AreEqual(userId, subscription.Id);
        Assert.AreEqual(expectedStartDate, subscription.StartDate);
        Assert.AreEqual(expectedEndDate, subscription.EndDate);
        Assert.IsTrue(subscription.IsActive);
    }

    [Test]
    public void GetExpiredSubscriptions_ReturnsOnlyExpiredOnes()
    {
        // Arrange
        var subscriptions = new List<SubscriptionService.Subscription>
        {
            new() { Id = 1, EndDate = _timeProvider.GetUtcNow().DateTime.AddDays(-1) }, // Expired
            new() { Id = 2, EndDate = _timeProvider.GetUtcNow().DateTime.AddDays(1) },  // Active
            new() { Id = 3, EndDate = _timeProvider.GetUtcNow().DateTime.AddDays(-5) }  // Expired
        };

        // Act
        var expired = _subscriptionService.GetExpiredSubscriptions(subscriptions);

        // Assert
        Assert.AreEqual(2, expired.Count);
        Assert.Contains(subscriptions[0], expired);
        Assert.Contains(subscriptions[2], expired);
    }

    [Test]
    public void GetTimeUntilExpiration_CalculatesCorrectly()
    {
        // Arrange
        var subscription = new SubscriptionService.Subscription
        {
            Id = 1,
            EndDate = _timeProvider.GetUtcNow().DateTime.AddDays(30)
        };

        // Act
        var timeUntil = _subscriptionService.GetTimeUntilExpiration(subscription);

        // Assert
        Assert.AreEqual(30, timeUntil.TotalDays);

        // Test with time advancement
        _timeProvider.AdvanceTime(TimeSpan.FromDays(10));
        var remainingTime = _subscriptionService.GetTimeUntilExpiration(subscription);
        Assert.AreEqual(20, remainingTime.TotalDays);
    }

    [Test]
    public void GetTimeUntilExpiration_ReturnsZeroForExpiredSubscription()
    {
        // Arrange
        var expiredSubscription = new SubscriptionService.Subscription
        {
            Id = 1,
            EndDate = _timeProvider.GetUtcNow().DateTime.AddDays(-1) // Already expired
        };

        // Act
        var timeUntil = _subscriptionService.GetTimeUntilExpiration(expiredSubscription);

        // Assert
        Assert.AreEqual(TimeSpan.Zero, timeUntil);
    }
}

Testing Time-Sensitive Business Logic

TimeProvider makes it easy to test complex business scenarios:

[TestFixture]
public class OrderServiceTests
{
    private FakeTimeProvider _timeProvider;
    private OrderService _orderService;

    [SetUp]
    public void Setup()
    {
        _timeProvider = new FakeTimeProvider(new DateTimeOffset(2024, 3, 15, 9, 0, 0, TimeSpan.Zero));
        _orderService = new OrderService(_timeProvider);
    }

    [Test]
    public void CreateOrder_UsesCurrentTime()
    {
        // Arrange
        var customer = new Customer { Id = 123 };
        var expectedTime = _timeProvider.GetUtcNow().DateTime;

        // Act
        var order = _orderService.CreateOrder(customer);

        // Assert
        Assert.AreEqual(expectedTime, order.CreatedAt);
        Assert.AreEqual(123, order.CustomerId);
    }

    [Test]
    public void IsOrderExpired_ReturnsFalseForNewOrder()
    {
        // Arrange
        var customer = new Customer { Id = 123 };
        var order = _orderService.CreateOrder(customer);
        var expirationPeriod = TimeSpan.FromHours(24);

        // Act
        var isExpired = _orderService.IsOrderExpired(order, expirationPeriod);

        // Assert
        Assert.IsFalse(isExpired);
    }

    [Test]
    public void IsOrderExpired_ReturnsTrueAfterExpirationPeriod()
    {
        // Arrange
        var customer = new Customer { Id = 123 };
        var order = _orderService.CreateOrder(customer);
        var expirationPeriod = TimeSpan.FromHours(24);

        // Act - advance time beyond expiration period
        _timeProvider.AdvanceTime(TimeSpan.FromHours(25));
        var isExpired = _orderService.IsOrderExpired(order, expirationPeriod);

        // Assert
        Assert.IsTrue(isExpired);
    }

    [Test]
    public void BusinessScenario_OrderExpiration()
    {
        // Test a complex business scenario
        var customer = new Customer { Id = 123 };

        // Create order at 9:00 AM
        var order = _orderService.CreateOrder(customer);
        Assert.AreEqual(new DateTime(2024, 3, 15, 9, 0, 0), order.CreatedAt);

        // Check at 2:00 PM (5 hours later) - should not be expired for 24-hour expiration
        _timeProvider.AdvanceTimeBy(hours: 5);
        Assert.IsFalse(_orderService.IsOrderExpired(order, TimeSpan.FromHours(24)));

        // Check next day at 10:00 AM (25 hours total) - should be expired
        _timeProvider.AdvanceTimeBy(hours: 20);
        Assert.IsTrue(_orderService.IsOrderExpired(order, TimeSpan.FromHours(24)));
    }
}

Migration from DateTime.Now to TimeProvider

If you have existing code using DateTime.Now, here's how to migrate:

// Before: Hard-coded DateTime dependency
public class LegacyAuditService
{
    public void LogUserAction(string action, int userId)
    {
        var logEntry = new AuditLog
        {
            Action = action,
            UserId = userId,
            Timestamp = DateTime.UtcNow // Hard dependency on system time
        };

        SaveLog(logEntry);
    }
}

// After: Testable with TimeProvider
public class ModernAuditService
{
    private readonly TimeProvider _timeProvider;

    public ModernAuditService(TimeProvider timeProvider)
    {
        _timeProvider = timeProvider ?? TimeProvider.System;
    }

    public void LogUserAction(string action, int userId)
    {
        var logEntry = new AuditLog
        {
            Action = action,
            UserId = userId,
            Timestamp = _timeProvider.GetUtcNow().DateTime // Testable dependency
        };

        SaveLog(logEntry);
    }

    private void SaveLog(AuditLog log) { /* Save to database */ }
}

Advanced TimeProvider Patterns

For sophisticated testing scenarios, you might need more advanced patterns:

public class AdvancedFakeTimeProvider : TimeProvider
{
    private DateTimeOffset _currentTime;
    private readonly List<ScheduledAction> _scheduledActions = new();

    public AdvancedFakeTimeProvider(DateTimeOffset startTime)
    {
        _currentTime = startTime;
    }

    private class ScheduledAction
    {
        public DateTimeOffset ExecuteAt { get; set; }
        public Action Action { get; set; }
    }

    public override DateTimeOffset GetUtcNow() => _currentTime;
    public override TimeZoneInfo LocalTimeZone => TimeZoneInfo.Utc;

    // Schedule actions to execute when time advances
    public void ScheduleAction(TimeSpan delay, Action action)
    {
        _scheduledActions.Add(new ScheduledAction
        {
            ExecuteAt = _currentTime.Add(delay),
            Action = action
        });
    }

    public void AdvanceTime(TimeSpan duration)
    {
        var newTime = _currentTime.Add(duration);

        // Execute any scheduled actions that should trigger
        var actionsToExecute = _scheduledActions
            .Where(a => a.ExecuteAt <= newTime && a.ExecuteAt > _currentTime)
            .OrderBy(a => a.ExecuteAt)
            .ToList();

        foreach (var scheduledAction in actionsToExecute)
        {
            _currentTime = scheduledAction.ExecuteAt;
            scheduledAction.Action();
        }

        _currentTime = newTime;

        // Remove executed actions
        _scheduledActions.RemoveAll(actionsToExecute.Contains);
    }

    public override ITimer CreateTimer(TimerCallback callback, object? state, TimeSpan dueTime, TimeSpan period)
    {
        return new AdvancedFakeTimer(callback, state, dueTime, period, this);
    }
}

Performance Considerations

TimeProvider adds minimal overhead in production while providing huge benefits for testing:

public class PerformanceComparison
{
    public void CompareTimeProviderPerformance()
    {
        const int iterations = 1_000_000;

        // Direct DateTime.UtcNow
        var sw = Stopwatch.StartNew();
        for (int i = 0; i < iterations; i++)
        {
            var time = DateTime.UtcNow;
        }
        sw.Stop();
        Console.WriteLine($"DateTime.UtcNow: {sw.ElapsedMilliseconds}ms");

        // TimeProvider.System
        sw.Restart();
        var timeProvider = TimeProvider.System;
        for (int i = 0; i < iterations; i++)
        {
            var time = timeProvider.GetUtcNow();
        }
        sw.Stop();
        Console.WriteLine($"TimeProvider.System: {sw.ElapsedMilliseconds}ms");

        // Results show minimal difference in production
    }
}

Chapter Summary

TimeProvider revolutionizes time-dependent code testing in .NET. It provides clean abstraction over system time, enables deterministic testing of time-sensitive logic, integrates seamlessly with dependency injection, and maintains excellent performance in production. Migrate existing DateTime.Now usage to TimeProvider for more maintainable and testable code.

Chapter Exercise

Build a CacheService class that uses TimeProvider for expiration logic:

1. Store cached items with expiration times
2. Automatically remove expired items when accessed
3. Provide methods to manually expire items before their natural expiration
4. Create comprehensive tests that verify expiration behavior without waiting for real time
5. Test edge cases like items expiring exactly at access time and bulk expiration scenarios

Include performance benchmarks comparing your cache with and without TimeProvider to verify there's no significant overhead.

---

Chapter 11: Best Practices for Storing Dates in Databases

Database date storage might seem straightforward, but it's where many applications fall apart. Store a datetime wrong, and you'll spend months debugging timezone-related issues, inconsistent reports, and user confusion. The database doesn't know about your application's timezone context - it just stores what you give it.

The fundamental rule is simple: always store UTC in databases. But implementing this correctly requires understanding Entity Framework Core configurations, handling timezone conversions at the right boundaries, and designing your data models to prevent timezone confusion.

The UTC Storage Principle

Store all timestamps as UTC in your database, then convert to user timezones only at the presentation layer :[1]

// Entity model - always UTC in database
public class Order
{
    public int Id { get; set; }
    public int CustomerId { get; set; }
    public DateTime CreatedAt { get; set; }      // Always UTC in database
    public DateTime? ShippedAt { get; set; }     // Always UTC in database  
    public DateTime? DeliveredAt { get; set; }   // Always UTC in database
    public string Status { get; set; }

    // Helper method for display purposes
    public DateTime GetCreatedAtInTimeZone(TimeZoneInfo timeZone)
    {
        return TimeZoneInfo.ConvertTimeFromUtc(CreatedAt, timeZone);
    }
}

// Repository layer - ensure UTC storage
public class OrderRepository
{
    private readonly AppDbContext _context;

    public OrderRepository(AppDbContext context)
    {
        _context = context;
    }

    public async Task<Order> CreateOrderAsync(int customerId)
    {
        var order = new Order
        {
            CustomerId = customerId,
            CreatedAt = DateTime.UtcNow, // Always store UTC
            Status = "Pending"
        };

        _context.Orders.Add(order);
        await _context.SaveChangesAsync();
        return order;
    }

    public async Task MarkOrderShippedAsync(int orderId)
    {
        var order = await _context.Orders.FindAsync(orderId);
        if (order != null)
        {
            order.ShippedAt = DateTime.UtcNow; // Always UTC
            order.Status = "Shipped";
            await _context.SaveChangesAsync();
        }
    }
}

Entity Framework Core Configuration

Configure EF Core to handle UTC properly across different database providers:

public class AppDbContext : DbContext
{
    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        // Configure DateTime properties to be stored as UTC
        modelBuilder.Entity<Order>(entity =>
        {
            entity.HasKey(e => e.Id);

            // Explicitly configure DateTime columns
            entity.Property(e => e.CreatedAt)
                .HasColumnType("datetime2") // SQL Server
                .IsRequired();

            entity.Property(e => e.ShippedAt)
                .HasColumnType("datetime2")
                .IsRequired(false);

            entity.Property(e => e.DeliveredAt)
                .HasColumnType("datetime2")
                .IsRequired(false);
        });

        // Configure for other entities
        ConfigureAuditableEntity<Customer>(modelBuilder);
        ConfigureAuditableEntity<Product>(modelBuilder);

        base.OnModelCreating(modelBuilder);
    }

    // Reusable configuration for entities with audit timestamps
    private void ConfigureAuditableEntity<T>(ModelBuilder modelBuilder) 
        where T : class, IAuditableEntity
    {
        modelBuilder.Entity<T>()
            .Property(e => e.CreatedAt)
            .HasDefaultValueSql("GETUTCDATE()"); // SQL Server UTC default

        modelBuilder.Entity<T>()
            .Property(e => e.UpdatedAt)
            .HasDefaultValueSql("GETUTCDATE()");
    }
}

// Interface for auditable entities
public interface IAuditableEntity
{
    DateTime CreatedAt { get; set; }
    DateTime? UpdatedAt { get; set; }
}

public class Customer : IAuditableEntity
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Email { get; set; }
    public DateTime CreatedAt { get; set; }    // Always UTC
    public DateTime? UpdatedAt { get; set; }   // Always UTC
}

Handling Different Database Providers

Different databases handle datetime storage differently. Configure appropriately:

public static class DatabaseDateTimeConfiguration
{
    public static void ConfigureForProvider(ModelBuilder modelBuilder, string providerName)
    {
        switch (providerName.ToLower())
        {
            case "microsoft.entityframeworkcore.sqlserver":
                ConfigureSqlServer(modelBuilder);
                break;

            case "npgsql.entityframeworkcore.postgresql":
                ConfigurePostgreSQL(modelBuilder);
                break;

            case "microsoft.entityframeworkcore.sqlite":
                ConfigureSQLite(modelBuilder);
                break;

            default:
                ConfigureGeneric(modelBuilder);
                break;
        }
    }

    private static void ConfigureSqlServer(ModelBuilder modelBuilder)
    {
        // SQL Server: Use datetime2 for better precision
        foreach (var entityType in modelBuilder.Model.GetEntityTypes())
        {
            foreach (var property in entityType.GetProperties())
            {
                if (property.ClrType == typeof(DateTime) || property.ClrType == typeof(DateTime?))
                {
                    property.SetColumnType("datetime2");
                }
            }
        }
    }

    private static void ConfigurePostgreSQL(ModelBuilder modelBuilder)
    {
        // PostgreSQL: Use timestamp without time zone (stores UTC)
        foreach (var entityType in modelBuilder.Model.GetEntityTypes())
        {
            foreach (var property in entityType.GetProperties())
            {
                if (property.ClrType == typeof(DateTime) || property.ClrType == typeof(DateTime?))
                {
                    property.SetColumnType("timestamp without time zone");
                }
            }
        }
    }

    private static void ConfigureSQLite(ModelBuilder modelBuilder)
    {
        // SQLite: Store as text in ISO 8601 format
        foreach (var entityType in modelBuilder.Model.GetEntityTypes())
        {
            foreach (var property in entityType.GetProperties())
            {
                if (property.ClrType == typeof(DateTime) || property.ClrType == typeof(DateTime?))
                {
                    property.SetColumnType("TEXT");
                }
            }
        }
    }

    private static void ConfigureGeneric(ModelBuilder modelBuilder)
    {
        // Generic configuration - let EF Core choose appropriate types
        // Add any cross-provider configuration here
    }
}

Automatic UTC Conversion with Interceptors

Use EF Core interceptors to automatically ensure UTC storage:

public class UtcDateTimeInterceptor : SaveChangesInterceptor
{
    public override InterceptionResult<int> SavingChanges(
        DbContextEventData eventData, 
        InterceptionResult<int> result)
    {
        ConvertToUtc(eventData.Context);
        return base.SavingChanges(eventData, result);
    }

    public override ValueTask<InterceptionResult<int>> SavingChangesAsync(
        DbContextEventData eventData, 
        InterceptionResult<int> result, 
        CancellationToken cancellationToken = default)
    {
        ConvertToUtc(eventData.Context);
        return base.SavingChangesAsync(eventData, result, cancellationToken);
    }

    private static void ConvertToUtc(DbContext? context)
    {
        if (context == null) return;

        foreach (var entry in context.ChangeTracker.Entries())
        {
            if (entry.Entity is IAuditableEntity auditableEntity)
            {
                switch (entry.State)
                {
                    case EntityState.Added:
                        auditableEntity.CreatedAt = EnsureUtc(auditableEntity.CreatedAt);
                        break;

                    case EntityState.Modified:
                        auditableEntity.UpdatedAt = DateTime.UtcNow;
                        break;
                }
            }

            // Convert all DateTime properties to UTC
            foreach (var property in entry.Properties)
            {
                if (property.CurrentValue is DateTime dateTime)
                {
                    property.CurrentValue = EnsureUtc(dateTime);
                }
            }
        }
    }

    private static DateTime EnsureUtc(DateTime dateTime)
    {
        return dateTime.Kind switch
        {
            DateTimeKind.Utc => dateTime,
            DateTimeKind.Local => dateTime.ToUniversalTime(),
            DateTimeKind.Unspecified => DateTime.SpecifyKind(dateTime, DateTimeKind.Utc),
            _ => dateTime
        };
    }
}

// Register the interceptor
public class AppDbContext : DbContext
{
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder.AddInterceptors(new UtcDateTimeInterceptor());
        base.OnConfiguring(optionsBuilder);
    }
}

Querying with Date Ranges

When querying date ranges, always work in UTC to avoid timezone confusion:

public class ReportService
{
    private readonly AppDbContext _context;

    public ReportService(AppDbContext context)
    {
        _context = context;
    }

    // ❌ Bad: Ambiguous - what timezone is this date range in?
    public async Task<List<Order>> GetOrdersByDateBad(DateTime startDate, DateTime endDate)
    {
        return await _context.Orders
            .Where(o => o.CreatedAt >= startDate && o.CreatedAt <= endDate)
            .ToListAsync();
    }

    // ✅ Good: Explicit UTC parameters
    public async Task<List<Order>> GetOrdersByDateRange(DateTime startUtc, DateTime endUtc)
    {
        // Ensure parameters are UTC
        if (startUtc.Kind != DateTimeKind.Utc || endUtc.Kind != DateTimeKind.Utc)
            throw new ArgumentException("Date parameters must be UTC");

        return await _context.Orders
            .Where(o => o.CreatedAt >= startUtc && o.CreatedAt <= endUtc)
            .ToListAsync();
    }

    // ✅ Better: Convert user's local date range to UTC for querying
    public async Task<List<Order>> GetOrdersForUserDateRange(
        DateOnly startDate, 
        DateOnly endDate, 
        TimeZoneInfo userTimeZone)
    {
        // Convert user's date range to UTC for database query
        var startUtc = TimeZoneInfo.ConvertTimeToUtc(
            startDate.ToDateTime(TimeOnly.MinValue), userTimeZone);
        var endUtc = TimeZoneInfo.ConvertTimeToUtc(
            endDate.ToDateTime(TimeOnly.MaxValue), userTimeZone);

        return await _context.Orders
            .Where(o => o.CreatedAt >= startUtc && o.CreatedAt <= endUtc)
            .ToListAsync();
    }
}

Working with DateOnly and TimeOnly in Databases

.NET 6+ DateOnly and TimeOnly types work well with modern EF Core versions:

public class Event
{
    public int Id { get; set; }
    public string Title { get; set; }
    public DateOnly EventDate { get; set; }      // Just the date
    public TimeOnly StartTime { get; set; }      // Just the time
    public TimeOnly? EndTime { get; set; }       // Optional end time
    public DateTime CreatedAtUtc { get; set; }   // Full timestamp in UTC
}

public class EventConfiguration : IEntityTypeConfiguration<Event>
{
    public void Configure(EntityTypeBuilder<Event> builder)
    {
        builder.HasKey(e => e.Id);

        // EF Core 6+ supports DateOnly and TimeOnly natively
        builder.Property(e => e.EventDate)
            .HasColumnType("date");  // SQL Server DATE type

        builder.Property(e => e.StartTime)
            .HasColumnType("time");  // SQL Server TIME type

        builder.Property(e => e.EndTime)
            .HasColumnType("time")
            .IsRequired(false);

        builder.Property(e => e.CreatedAtUtc)
            .HasColumnType("datetime2")
            .HasDefaultValueSql("GETUTCDATE()");
    }
}

public class EventService
{
    private readonly AppDbContext _context;

    public EventService(AppDbContext context)
    {
        _context = context;
    }

    public async Task<Event> CreateEventAsync(string title, DateOnly date, TimeOnly startTime, TimeOnly? endTime = null)
    {
        var eventEntity = new Event
        {
            Title = title,
            EventDate = date,
            StartTime = startTime,
            EndTime = endTime,
            CreatedAtUtc = DateTime.UtcNow  // Always UTC for audit trail
        };

        _context.Events.Add(eventEntity);
        await _context.SaveChangesAsync();
        return eventEntity;
    }

    public async Task<List<Event>> GetEventsForDateAsync(DateOnly date)
    {
        return await _context.Events
            .Where(e => e.EventDate == date)
            .OrderBy(e => e.StartTime)
            .ToListAsync();
    }
}

Handling Legacy Data Migration

When migrating existing databases with timezone issues:

public class TimezoneMigrationService
{
    private readonly AppDbContext _context;
    private readonly ILogger<TimezoneMigrationService> _logger;

    public TimezoneMigrationService(AppDbContext context, ILogger<TimezoneMigrationService> logger)
    {
        _context = context;
        _logger = logger;
    }

    // Migration method to convert local times to UTC
    public async Task MigrateLocalTimesToUtcAsync(TimeZoneInfo sourceTimeZone)
    {
        _logger.LogInformation("Starting timezone migration from {TimeZone} to UTC", sourceTimeZone.Id);

        var batchSize = 1000;
        var totalMigrated = 0;

        while (true)
        {
            // Process in batches to avoid memory issues
            var orders = await _context.Orders
                .Where(o => o.CreatedAt.Kind == DateTimeKind.Unspecified) // Find unmigrated records
                .Take(batchSize)
                .ToListAsync();

            if (!orders.Any()) break;

            foreach (var order in orders)
            {
                try
                {
                    // Convert from source timezone to UTC
                    var utcTime = TimeZoneInfo.ConvertTimeToUtc(
                        DateTime.SpecifyKind(order.CreatedAt, DateTimeKind.Unspecified), 
                        sourceTimeZone);

                    order.CreatedAt = utcTime;

                    // Migrate other timestamp fields
                    if (order.ShippedAt.HasValue)
                    {
                        order.ShippedAt = TimeZoneInfo.ConvertTimeToUtc(
                            DateTime.SpecifyKind(order.ShippedAt.Value, DateTimeKind.Unspecified), 
                            sourceTimeZone);
                    }

                    if (order.DeliveredAt.HasValue)
                    {
                        order.DeliveredAt = TimeZoneInfo.ConvertTimeToUtc(
                            DateTime.SpecifyKind(order.DeliveredAt.Value, DateTimeKind.Unspecified), 
                            sourceTimeZone);
                    }
                }
                catch (Exception ex)
                {
                    _logger.LogError(ex, "Failed to migrate order {OrderId}", order.Id);
                }
            }

            await _context.SaveChangesAsync();
            totalMigrated += orders.Count;

            _logger.LogInformation("Migrated {BatchCount} orders, total: {TotalCount}", 
                orders.Count, totalMigrated);
        }

        _logger.LogInformation("Timezone migration completed. Total migrated: {TotalCount}", totalMigrated);
    }
}

Common Database Storage Anti-Patterns

Storing local time without timezone context:

// ❌ Bad: No way to know what timezone this is
public class BadOrder
{
    public DateTime CreatedAt { get; set; } // Local time? UTC? Unknown?
}

// ✅ Good: Always UTC, convert at presentation
public class GoodOrder  
{
    public DateTime CreatedAtUtc { get; set; } // Clear UTC intent
}

Using DATETIME instead of DATETIME2:

// ❌ Bad: Lower precision, potential rounding issues
entity.Property(e => e.CreatedAt).HasColumnType("datetime");

// ✅ Good: Higher precision, better for precise timestamps
entity.Property(e => e.CreatedAt).HasColumnType("datetime2");

Storing timezone information in the wrong place:

// ❌ Bad: Storing timezone per record is usually wrong
public class BadEvent
{
    public DateTime StartTime { get; set; }
    public string TimeZone { get; set; } // Usually indicates design problem
}

// ✅ Good: Store user timezone preference separately
public class GoodEvent
{
    public DateTime StartTimeUtc { get; set; } // Always UTC in database
}

public class UserProfile
{
    public string PreferredTimeZoneId { get; set; } // User's display preference
}

Chapter Summary

Always store timestamps as UTC in databases to eliminate timezone ambiguity. Configure Entity Framework Core appropriately for your database provider, use interceptors for automatic UTC conversion, and design clear data models that separate storage (UTC) from presentation (user timezone). When querying date ranges, convert user input to UTC before database queries.

Chapter Exercise

Create a complete data layer for a blog system that handles:

1. Posts with creation and publication timestamps (UTC storage)
2. Comments with threaded replies and timestamps
3. User timezone preferences for display
4. Date-range queries for posts (e.g., "posts from last week" in user's timezone)
5. Automatic audit trail timestamps
6. Migration script for converting existing local timestamps to UTC

Include comprehensive tests that verify timezone handling works correctly across different user timezones and database providers.