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Core Java: Date and Time

Hamza Belmellouki
Hamza (@HamzaLovesJava) is a self-taught developer and Java enthusiast. In his free time, he likes to blog about various technical topics at https://hamza-jvm.me.
・7 min read

Since Java 8, Oracle completely rebuilt its Date/Time API. The new API is supposed to replace the old one.

The new API which is located in the java.time package is thread-safe because most of the new classes are immutable, meaning that, after the object is constructed, it cannot be modified. This is especially useful when working in a multi-threaded environment where issues like thread interference and data corruption cannot happen thanks to immutability.

This article shows how to work with the new API.

Creating Dates and Times

Java lets us create dates and times using static factory methods. Note that you cannot create date and time objects using a constructor (you can do so with the old API, but you shouldn’t) because it is made private.

Creating dates and times is straightforward; you’ll notice a pattern for creating dates and times:

LocalDate

You create a LocalDate by using one of its static factory methods:

LocalDate now = LocalDate.now();
System.out.println(now);// 2020-02-21

LocalDate date = LocalDate.of(2020, 1, 23);
System.out.println(date);// 2020-01-23

LocalDate date2 = LocalDate.of(2020, Month.JANUARY, 23);
System.out.println(date2);// 2020-01-23
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Note that month indexes are one-based.

LocalTime

Similarly, you create a LocalTime object like so:

LocalTime now = LocalTime.now();
System.out.println(now);// 12:31:56.186

LocalTime midnight = LocalTime.of(23, 0);
System.out.println(midnight); // 23:00
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The second line output a toString representation of LocalTime, which represents [hour:minutes:seconds.milliseconds]

LocalDateTime

The LocalDateTime class represents date and time combined. You can create it with:

LocalDateTime now = LocalDateTime.now();
System.out.println(now);// 2020-02-21T12:46:09.950

LocalDateTime newYear = LocalDateTime.of(LocalDate.of(2021, 12, 31), LocalTime.of(11, 59));
System.out.println(newYear);// 2021-12-31T11:59
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Note that in the output date and time are separated with a T.

ZonedDateTime

Use this class if you want to express in a date and time in a specific timezone. for example:

ZonedDateTime usPacific = ZonedDateTime.now(ZoneId.of("US/Pacific"));
System.out.println(usPacific);// 2020-02-22T02:53:46.774455-08:00[US/Pacific]

ZonedDateTime now = ZonedDateTime.now();// 2020-02-22T11:53:46.778363+01:00[Africa/Casablanca]
System.out.println(now);
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The format of the output consists of LocalDateTime followed by the ZoneOffset.

Manipulating Dates and Times

LocalDate date = LocalDate.of(2020, Month.JANUARY, 20);
System.out.println(date);// 2020–01–20

date = date.plusDays(1);
System.out.println(date);// 2020-01-21

date = date.plusWeeks(3);
System.out.println(date);// 2020-02-11

date = date.plusMonths(4);
System.out.println(date);// 2020-06-11

date = date.plusYears(10);
System.out.println(date);// 2030-06-11
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You these methods return a LocalDate object. Therefore, you can chain them:

LocalDate date = LocalDate.of(2020, Month.JANUARY, 20)
        .plusDays(1).plusWeeks(3)
        .plusMonths(4).plusYears(10);
System.out.println(date);// 2030-06-11
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Using the same pattern, you can subtract dates/times from LocalDate, LocalDateTime, LocalTime and ZonedDateTime using minus###() method.

Periods and Durations

Periods

You create a period from the Period class. This class represents the amount of time in years, months, and days. These examples demonstrate the typical ways you would create a Period:

Period threeDays = Period.ofDays(3);
System.out.println(threeDays);// P3D

Period threeWeeks = Period.ofWeeks(3);
System.out.println(threeWeeks);// P21D

Period threeMonths = Period.ofMonths(3);
System.out.println(threeMonths);// P3M

Period threeYears = Period.ofYears(3);
System.out.println(threeYears);// P3Y

Period threeYearsAndFourMonthsAndTwoDays = Period.of(3, 4, 2);
System.out.println(threeYearsAndFourMonthsAndTwoDays);// P3Y4M2D
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These static factory methods are self-explanatory; they create an immutable Period instance.

In the output, the letter P stands for Period, Y for years, M for months, and D for days.

Note that you cannot chain methods as you’ve seen in the LocalDate example when you create Period because these methods are static, If you chain them you’ll get unexpected behavior:

Period oneWeekAndADay = Period.ofDays(1).ofWeeks(1);
System.out.println(oneWeekAndADay); // unexpected result: P7D
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Remember that a Period cannot be be used with some objects. Let’s look at some code:

Period period = Period.ofDays(1);
LocalTime time = LocalTime.of(6, 15);
time.plus(period); // UnsupportedTemporalTypeException
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Durations

You create a duration form the Duration class. This class represents the amount of time in seconds and nanoseconds. It can also be expressed using other duration-based units, such as minutes and hours. These examples demonstrate the typical ways you would create a Duration:

Duration oneNano = Duration.ofNanos(1);
System.out.println(oneNano);// PT0.000000001S

Duration oneMilli = Duration.ofMillis(1);
System.out.println(oneMilli);// PT0.001S

Duration oneSeconds = Duration.ofSeconds(1);
System.out.println(oneSeconds);// PT1S

Duration oneMinute = Duration.ofMinutes(1);
System.out.println(oneMinute);// PT1M

Duration oneHour = Duration.ofHours(1);
System.out.println(oneHour);// PT1H

Duration oneDay = Duration.ofDays(1);
System.out.println(oneDay);// PT24H
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Alternatively, you can create a Duration using the following method:

Duration fiveHours = Duration.of(5, ChronoUnit.HOURS);
System.out.println(fiveHours);
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This method takes 5 as an amount and a unit that the duration is measured in.

Working with Instants

You create an instant from the Instant class. This class represents a single instantaneous point on the time-line in the GMT since January 1, 1970 (1970–01–01T00:00:00Z), a.k.a the EPOCH. It may come in handy when you want to record event timestamps in the program. These examples demonstrate the typical ways you would create an Instant:

Instant now = Instant.now();
System.out.println(now);//2020-02-25T11:14:46.032856Z

ZonedDateTime zonedDateTime = ZonedDateTime.now(ZoneId.of("US/Eastern"));
System.out.println(zonedDateTime);// 2020-02-25T06:27:27.572624-05:00[US/Eastern]
Instant now2 = Instant.from(zonedDateTime);
System.out.println(now2);// 2020-02-25T11:14:46.055857Z

Instant instant = Instant.parse("2010-01-20T11:33:45Z");
System.out.println(instant);// 2010-01-20T11:33:45Z

Instant epoch = Instant.ofEpochMilli(0);
System.out.println(epoch);// 1970-01-01T00:00:00Z
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Note that the output of toString follows the ISO-8601 standard.

As you can see, when Java invoked Instant.from(zonedDateTime); it created an Instant object from ZonedDateTime object and converted the time from US/Eastern timezone to GMT.

Likewise, this class provides various ways to operate on instants. For example:

Instant tenMinutesLater = Instant.now().plus(10, ChronoUnit.MINUTES);
System.out.println(tenMinutesLater);// print 10 minutes later from the current time
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Parsing and Formatting

Formating

The JDK provides a new API to parse and format Temporal-based objects, using the DateTimeFormatter from the java.time.format package we can parse and format dates and times. Similar to most other new Date/Time API classes, DateTimeFormatter is immutable thus thread-safe.

The format method is provided by those classes for formatting temporal-based objects for display. For example, this snippet of code format date and time using a predefined formatter:

LocalDate date = LocalDate.of(2020, Month.MARCH, 17);
LocalTime time = LocalTime.of(9, 15, 45);
LocalDateTime dateTime = LocalDateTime.of(date, time);

System.out.println(
dateTime.format(DateTimeFormatter.ISO_LOCAL_DATE_TIME));// 2020-03-17T09:15:45

System.out.println(
date.format(DateTimeFormatter.ISO_LOCAL_DATE));// 2020-03-17

System.out.println(
time.format(DateTimeFormatter.ISO_LOCAL_TIME));// 09:15:45

DateTimeFormatter shortF = DateTimeFormatter.ofLocalizedDateTime(FormatStyle.SHORT);
System.out.println(shortF.format(dateTime));// 3/17/20, 9:15 AM

DateTimeFormatter mediumF = DateTimeFormatter.ofLocalizedDateTime(FormatStyle.MEDIUM);
System.out.println(mediumF.format(dateTime));// Mar 17, 2020, 9:15:45 AM
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We can also define a custom formatter object using DateTimeFormatter.ofPattern method:

LocalDate date = LocalDate.of(2020, Month.MARCH, 17);
LocalTime time = LocalTime.of(9, 15, 45);
LocalDateTime dateTime = LocalDateTime.of(date, time);

DateTimeFormatter f = DateTimeFormatter.ofPattern("dd-MMMM-yyyy | hh:mm");
System.out.println(dateTime.format(f));// 17-March-2020 | 09:15
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Make sure to take a look at the reference documentation if you want to know more about the syntax used in the ofPattern method argument.

Parsing

Now you know how to convert Temporal-based classes into strings, let’s see how we can convert into the other direction using the parse method:

DateTimeFormatter f = DateTimeFormatter.ofPattern("dd MM yyyy");
LocalDate date = LocalDate.parse("25 03 2020", f);
LocalTime time = LocalTime.parse("09:30");

System.out.println(date);// 2020-03-25
System.out.println(time);// 09:30
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LocalDate.parse(“25 03 2020“, f) returns a LocalDate by parsing the text string using a custom formatter object. While LocalTime.parse(“09:30”) parses the text string and returns a LocalTime using the default formatter (DateTimeFormatter.ISO_LOCAL_TIME)

A Peek on Dates and Times Before Java 8

Before Java 8, developers used the Date class, which represents the date and time altogether. There was no way to get a date or time separately, and it was all bundled in the Date class. Developers should not use this class anymore, as you’ve seen, there is a better way. This class exists to support backward compatibility.

Another disadvantage of the Date class is that it’s mutable. Therefore, you need to synchronize access to instances of this class when they’re accessed from multiple threads to avoid data corruption and unexpected behavior.

You may encounter the old Date API in legacy projects. You create a date by calling its constructor: Date date = new Date() , date refers to the current date/time. You can specify a specific date using the Calendar class:

Calendar cal = Calendar.getInstance();
cal.set(2020, 0, 23);
Date d = cal.getTime();
System.out.println(d);// Wed Jan 01 11:36:37 WET 2020
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You can see how much verbose this old API compared to the new one. Beware that Month indexes are zero-based instead of one-based, which is confusing. The new API’s indexes are one-based.

Wrap Up

In this post, I attempted to demonstrate why you should work with the new Date/Time API, how to work with it, how to format and parse dates and times. And, peeked at how dates/times were handled before Java 8.

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