DEV Community: Sam

Cron Expressions Explained: A Beginner-Friendly Guide with Examples

Sam — Fri, 11 Sep 2026 15:45:50 +0000

If you have ever needed to run a task automatically every few minutes, every day, or every Monday morning, you have probably come across cron expressions.

Cron is widely used for scheduling recurring tasks on Linux servers, backend systems, CI/CD jobs, and automation workflows.

A cron expression may look confusing at first:

*/5 * * * *

But once you understand the five fields, it becomes much easier.

What Is a Cron Expression?

A standard Unix cron expression normally contains five fields:

* * * * *
│ │ │ │ │
│ │ │ │ └── Day of week
│ │ │ └──── Month
│ │ └────── Day of month
│ └──────── Hour
└────────── Minute

Each field controls when the command should run.

For example:

0 9 * * *

means:

Run every day at 9:00 AM.

Understanding the Five Fields

The standard structure is:

minute hour day-of-month month day-of-week

For example:

30 14 * * *

means:

Minute: 30
Hour: 14
Day of month: every day
Month: every month
Day of week: every day

So the job runs every day at:

14:30

or:

2:30 PM

The Asterisk `*`

The * means:

every possible value

For example:

* * * * *

means:

Run every minute.

This is usually not something you want in production unless the task is designed to run that frequently.

Run Every 5 Minutes

*/5 * * * *

The */5 means:

every 5 minutes

So the job runs at:

minutes past every hour.

Run Every Hour

0 * * * *

This means:

Run at minute 0 of every hour.

Examples:

Run Every Day at Midnight

0 0 * * *

This runs every day at:

00:00

A common use case is a nightly cleanup or backup task.

Run Every Day at 9 AM

0 9 * * *

This means:

Minute: 0
Hour: 9

So it runs every day at:

09:00

Run Every Monday at 9 AM

0 9 * * 1

In many cron implementations:

0 = Sunday
1 = Monday
2 = Tuesday
3 = Wednesday
4 = Thursday
5 = Friday
6 = Saturday

So:

0 9 * * 1

means:

Run every Monday at 9:00 AM.

Run on the First Day of Every Month

0 0 1 * *

This means:

Run at midnight on the first day of every month.

This can be useful for:

monthly reports
billing jobs
cleanup tasks
data archival

Run Every 15 Minutes During Working Hours

You can combine values:

*/15 9-17 * * 1-5

This means:

Every 15 minutes
Between 9 AM and 5 PM
Monday through Friday

This is a good example of how cron expressions can become powerful while still staying compact.

Common Cron Symbols

A few symbols appear frequently:

*     every value
,     multiple values
-     range
/     step value

Examples:

1,3,5

means:

1, 3, and 5

Example:

1-5

means:

1 through 5

Example:

*/10

means:

every 10 units

Common Cron Examples

*/5 * * * *

Every 5 minutes.

0 * * * *

Every hour.

0 0 * * *

Every day at midnight.

0 9 * * 1-5

Every weekday at 9 AM.

0 0 1 * *

First day of every month.

30 18 * * 5

Every Friday at 6:30 PM.

A Common Mistake: Cron Timezones

Cron expressions themselves normally describe a schedule, but the actual execution timezone depends on the system running the cron job.

For example:

0 9 * * *

means 9 AM according to the timezone configured on the server or scheduler.

This can cause unexpected behavior when:

your server is using UTC
your users are in another timezone
daylight saving time changes
your cloud platform uses a different timezone

Always check the timezone used by your scheduler.

Cron Syntax Can Vary

Not every cron implementation behaves exactly the same.

Traditional Unix cron commonly uses five fields:

* * * * *

But some systems add an extra field for seconds:

* * * * * *

Other platforms may have different rules for day-of-week values or special syntax.

So always check the documentation for the platform you are using.

A Tool I Built for Testing Cron Expressions

While working with cron expressions, I wanted a quick way to understand a schedule and preview upcoming run times.

So I built a Cron Expression Tester as part of my project, YBS — Your Browser Suite.

👉 Try the Cron Expression Tester

It helps you:

test cron expressions
understand each field
preview upcoming run times
catch scheduling mistakes quickly

I also built a Cron Expression Generator for creating cron expressions using simple controls:

👉 Try the Cron Expression Generator

Final Thoughts

Cron expressions can look cryptic at first, but most schedules come down to understanding these five fields:

minute hour day-of-month month day-of-week

Once you get comfortable with *, ranges, lists, and step values, cron becomes a very useful tool for automation.

What cron schedule do you use most often?

Share your examples in the comments. 👇

Unix Timestamps Explained: A Practical Guide for Developers

Sam — Thu, 10 Sep 2026 20:32:00 +0000

If you work with APIs, logs, databases, authentication tokens, or scheduled jobs, you have probably seen numbers like this:

1789056000

That number can represent a date and time using a Unix timestamp.

What Is a Unix Timestamp?

A Unix timestamp represents the number of seconds that have passed since:

January 1, 1970 00:00:00 UTC

This point in time is known as the Unix Epoch.

For example:

represents:

1970-01-01 00:00:00 UTC

As time passes, the Unix timestamp increases.

Seconds vs Milliseconds

One common source of bugs is confusing seconds with milliseconds.

A Unix timestamp in seconds may look like:

1789056000

The same type of value in milliseconds may look like:

1789056000000

JavaScript's Date API normally works with milliseconds.

For example:

const timestamp = 1789056000;

const date = new Date(timestamp * 1000);

console.log(date);

We multiply the timestamp by 1000 because:

1 second = 1000 milliseconds

Get the Current Unix Timestamp in JavaScript

Getting the current Unix timestamp is simple:

const timestamp = Math.floor(Date.now() / 1000);

console.log(timestamp);

Date.now() returns the current time in milliseconds.

Dividing it by 1000 converts it to seconds.

Math.floor() removes the decimal portion.

Convert a JavaScript Date to Unix Time

You can also convert any JavaScript Date object into a Unix timestamp.

const date = new Date("2026-09-11T10:00:00Z");

const timestamp = Math.floor(date.getTime() / 1000);

console.log(timestamp);

Here, getTime() returns milliseconds, so we divide the result by 1000.

Convert Unix Time Back to a Date

You can convert a Unix timestamp back into a readable JavaScript date:

const timestamp = 1789056000;

const date = new Date(timestamp * 1000);

console.log(date.toISOString());

This is especially useful when debugging API responses, database records, or logs.

Where Are Unix Timestamps Used?

Unix timestamps appear in many parts of software development:

REST APIs
JWT tokens
Database records
Server logs
Scheduled jobs
Caching systems
Authentication sessions
Analytics events

For example, a JWT payload might contain:

{
  "iat": 1789056000,
  "exp": 1789059600
}

Here:

iat means Issued At.

exp means Expiration Time.

These values make it easy for applications to determine when a token was created and when it should expire.

Why Are Unix Timestamps Useful?

One major advantage of Unix timestamps is that they represent a specific point in time.

For example, a value like:

11 September 2026, 3:30 PM

can become confusing because the timezone is unclear.

But a Unix timestamp represents the same moment regardless of where the user is located.

Your application can then convert that timestamp into the user's local timezone when displaying it.

This makes Unix timestamps especially useful for:

APIs, distributed applications, databases, authentication systems, and server logs.

A Small Tool I Built

While working with timestamps, I often needed a quick way to convert between Unix timestamps and readable dates.

Opening the browser console every time worked, but it wasn't very convenient.

So I built a Unix Timestamp Converter as part of a project I'm working on called YBS — Your Browser Suite.

👉 Try the Unix Timestamp Converter

It lets you quickly convert between Unix timestamps and readable date/time values directly in your browser.

What Is YBS?

YBS — Your Browser Suite is a collection of small browser-based utilities for common developer tasks.

It currently includes tools for things such as:

JSON formatting
Regex testing
Cron expressions
JWT decoding
Diff checking
Base64 encoding
Unix timestamp conversion
YAML viewing
Word and character counting
QR code generation

No signup is required.

I'm continuing to improve the project and add useful tools over time.

What Should I Build Next?

I'm curious what other developers repeatedly search for while coding.

What small developer tool do you wish you had available instantly in your browser?

Let me know in the comments. 👇

DEV Community: Sam

Cron Expressions Explained: A Beginner-Friendly Guide with Examples

What Is a Cron Expression?

Understanding the Five Fields

The Asterisk *

Run Every 5 Minutes

Run Every Hour

Run Every Day at Midnight

Run Every Day at 9 AM

Run Every Monday at 9 AM

Run on the First Day of Every Month

Run Every 15 Minutes During Working Hours

Common Cron Symbols

Common Cron Examples

A Common Mistake: Cron Timezones

Cron Syntax Can Vary

A Tool I Built for Testing Cron Expressions

Final Thoughts

Unix Timestamps Explained: A Practical Guide for Developers

What Is a Unix Timestamp?

Seconds vs Milliseconds

Get the Current Unix Timestamp in JavaScript

Convert a JavaScript Date to Unix Time

Convert Unix Time Back to a Date

Where Are Unix Timestamps Used?

Why Are Unix Timestamps Useful?

A Small Tool I Built

What Is YBS?

What Should I Build Next?

The Asterisk `*`