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1. Overview

In this tutorial, we'll review the Java 8 DateTimeFormatter class and its formatting patterns. We'll also discuss possible use cases for this class.

We can use DateTimeFormatter to uniformly format dates and times in an app with predefined or user-defined patterns.

2. DateTimeFormatter With Predefined Instances

DateTimeFormatter comes with multiple predefined date/time formats that follow ISO and RFC standards. For example, we can use the ISO_LOCAL_DATE instance to parse a date such as ‘2018-03-09':

DateTimeFormatter.ISO_LOCAL_DATE.format(LocalDate.of(2018, 3, 9));

To parse a date with an offset, we can use ISO_OFFSET_DATE to get an output like ‘2018-03-09-03:00':

DateTimeFormatter.ISO_OFFSET_DATE.format(LocalDate.of(2018, 3, 9).atStartOfDay(ZoneId.of("UTC-3")));

Most of the predefined instances of the DateTimeFormatter class are focused on the ISO-8601 standard. ISO-8601 is an international standard for date and time formatting.

There is, however, one different predefined instance that parses RFC-1123, Requirement for Internet Hosts, published by the IETF:

DateTimeFormatter.RFC_1123_DATE_TIME.format(LocalDate.of(2018, 3, 9).atStartOfDay(ZoneId.of("UTC-3")));

This snippet generates ‘Fri, 9 Mar 2018 00:00:00 -0300.

Sometimes we have to manipulate the date we receive as a String of a known format. For this, we can make use of the parse() method:

LocalDate.from(DateTimeFormatter.ISO_LOCAL_DATE.parse("2018-03-09")).plusDays(3);

The result of this code snippet is a LocalDate representation for March 12th, 2018.

3. DateTimeFormatter With FormatStyle

Sometimes we may want to print dates in a human-readable way.

In such cases, we may use java.time.format.FormatStyle enum (FULL, LONG, MEDIUM, SHORT) values with our DateTimeFormatter:

LocalDate anotherSummerDay = LocalDate.of(2016, 8, 23);
System.out.println(DateTimeFormatter.ofLocalizedDate(FormatStyle.FULL).format(anotherSummerDay));
System.out.println(DateTimeFormatter.ofLocalizedDate(FormatStyle.LONG).format(anotherSummerDay));
System.out.println(DateTimeFormatter.ofLocalizedDate(FormatStyle.MEDIUM).format(anotherSummerDay));
System.out.println(DateTimeFormatter.ofLocalizedDate(FormatStyle.SHORT).format(anotherSummerDay));

The output of these different formatting styles of the same date are:

Tuesday, August 23, 2016
August 23, 2016
Aug 23, 2016
8/23/16

We may also use predefined formatting styles for date and time. To use FormatStyle with time, we have to use the ZonedDateTime instance, otherwise, a DateTimeException will be thrown:

LocalDate anotherSummerDay = LocalDate.of(2016, 8, 23);
LocalTime anotherTime = LocalTime.of(13, 12, 45);
ZonedDateTime zonedDateTime = ZonedDateTime.of(anotherSummerDay, anotherTime, ZoneId.of("Europe/Helsinki"));
System.out.println(
  DateTimeFormatter.ofLocalizedDateTime(FormatStyle.FULL)
  .format(zonedDateTime));
System.out.println(
  DateTimeFormatter.ofLocalizedDateTime(FormatStyle.LONG)
  .format(zonedDateTime));
System.out.println(
  DateTimeFormatter.ofLocalizedDateTime(FormatStyle.MEDIUM)
  .format(zonedDateTime));
System.out.println(
  DateTimeFormatter.ofLocalizedDateTime(FormatStyle.SHORT)
  .format(zonedDateTime));

Note that we used the ofLocalizedDateTime() method of DateTimeFormatter this time.

The output we get is:

Tuesday, August 23, 2016 1:12:45 PM EEST
August 23, 2016 1:12:45 PM EEST
Aug 23, 2016 1:12:45 PM
8/23/16 1:12 PM

We can also use FormatStyle to parse a date time String, converting it to ZonedDateTime, for example.

We can then use the parsed value to manipulate the date and time variable:

ZonedDateTime dateTime = ZonedDateTime.from(
  DateTimeFormatter.ofLocalizedDateTime(FormatStyle.FULL)
    .parse("Tuesday, August 23, 2016 1:12:45 PM EET"));
System.out.println(dateTime.plusHours(9));

The output of this snippet is “2016-08-23T22:12:45+03:00[Europe/Bucharest].” Notice that the time has changed to “22:12:45.”

4. DateTimeFormatter With Custom Formats

Predefined and built-in formatters and styles can cover a lot of situations. However, sometimes we need to format a date and time somewhat differently. This is when custom formatting patterns come into play.

4.1. DateTimeFormatter for Date

Suppose we want to present a java.time.LocalDate object using a regular European format like 31.12.2018. To do this, we could call the factory method DateTimeFormatter.ofPattern(“dd.MM.yyyy”).

This will create an appropriate DateTimeFormatter instance that we can use to format our date:

String europeanDatePattern = "dd.MM.yyyy";
DateTimeFormatter europeanDateFormatter = DateTimeFormatter.ofPattern(europeanDatePattern);
System.out.println(europeanDateFormatter.format(LocalDate.of(2016, 7, 31)));

The output of this code snippet will be “31.07.2016.”

There are many different pattern letters that we can use to create a format for dates that will suit our needs:

  Symbol  Meaning                     Presentation      Examples
  ------  -------                     ------------      -------
   u       year                        year              2004; 04
   y       year-of-era                 year              2004; 04
   M/L     month-of-year               number/text       7; 07; Jul; July; J
   d       day-of-month                number            10

This is an extract of the official Java documentation to DateTimeFormatter class.

The number of letters in the pattern format is significant.

If we use a two-letter pattern for the month, we'll get a two-digit month representation. If the month number is less than 10, it will be padded with a zero. When we don't need the mentioned padding with zeroes, we can use a one-letter pattern “M,” which will show January as “1.”

If we happen to use a four-letter pattern for the month, “MMMM,” then we'll get a “full form” representation. In our example, it would be “July.” A five-letter pattern, “MMMMM,” will make the formatter use the “narrow form.” In our case, “J” would be used.

Likewise, custom formatting patterns can also be used to parse a String that holds a date:

DateTimeFormatter europeanDateFormatter = DateTimeFormatter.ofPattern("dd.MM.yyyy");
System.out.println(LocalDate.from(europeanDateFormatter.parse("15.08.2014")).isLeapYear());

This code snippet checks whether the date “15.08.2014” is one of a leap year, which it isn't.

4.2. DateTimeFormatter for Time

There are also pattern letters that can be used for time patterns:

  Symbol  Meaning                     Presentation      Examples
  ------  -------                     ------------      -------
   H       hour-of-day (0-23)          number            0
   m       minute-of-hour              number            30
   s       second-of-minute            number            55
   S       fraction-of-second          fraction          978
   n       nano-of-second              number            987654321

It's quite simple to use DateTimeFormatter to format a java.time.LocalTime instance. Suppose we want to show time (hours, minutes and seconds) delimited with a colon:

String timeColonPattern = "HH:mm:ss";
DateTimeFormatter timeColonFormatter = DateTimeFormatter.ofPattern(timeColonPattern);
LocalTime colonTime = LocalTime.of(17, 35, 50);
System.out.println(timeColonFormatter.format(colonTime));

This will generate output “17:35:50.

If we want to add milliseconds to the output, we should add “SSS” to the pattern:

String timeColonPattern = "HH:mm:ss SSS";
DateTimeFormatter timeColonFormatter = DateTimeFormatter.ofPattern(timeColonPattern);
LocalTime colonTime = LocalTime.of(17, 35, 50).plus(329, ChronoUnit.MILLIS);
System.out.println(timeColonFormatter.format(colonTime));

This gives us the output “17:35:50 329.

Note that “HH” is an hour-of-day pattern that generates the output of 0-23. When we want to show AM/PM, we should use lower-case “hh” for hours and add an “a” pattern:

String timeColonPattern = "hh:mm:ss a";
DateTimeFormatter timeColonFormatter = DateTimeFormatter.ofPattern(timeColonPattern);
LocalTime colonTime = LocalTime.of(17, 35, 50);
System.out.println(timeColonFormatter.format(colonTime));

The generated output is “05:35:50 PM.

We may want to parse a time String with our custom formatter and check if it's before noon:

DateTimeFormatter timeFormatter = DateTimeFormatter.ofPattern("hh:mm:ss a");
System.out.println(LocalTime.from(timeFormatter.parse("12:25:30 AM")).isBefore(LocalTime.NOON));

The output of this last snippet shows that the given time is actually before noon.

4.3. DateTimeFormatter for Time Zones

Often we want to see a time zone of some specific date-time variable. If we use New York-based date-time (UTC -4), we can use “z” pattern-letter for time-zone name:

String newYorkDateTimePattern = "dd.MM.yyyy HH:mm z";
DateTimeFormatter newYorkDateFormatter = DateTimeFormatter.ofPattern(newYorkDateTimePattern);
LocalDateTime summerDay = LocalDateTime.of(2016, 7, 31, 14, 15);
System.out.println(newYorkDateFormatter.format(ZonedDateTime.of(summerDay, ZoneId.of("UTC-4"))));

This will generate the output “31.07.2016 14:15 UTC-04:00.”

We can parse date time strings with time zones just like we did earlier:

DateTimeFormatter zonedFormatter = DateTimeFormatter.ofPattern("dd.MM.yyyy HH:mm z");
System.out.println(ZonedDateTime.from(zonedFormatter.parse("31.07.2016 14:15 GMT+02:00")).getOffset().getTotalSeconds());

The output of this code is “7200” seconds, or 2 hours, as we'd expect.

We have to make sure that we provide a correct date time String to the parse() method. If we pass “31.07.2016 14:15” without a time zone to the zonedFormatter from the last code snippet, we'll get a DateTimeParseException.

4.4. DateTimeFormatter for Instant

DateTimeFormatter comes with a great ISO instant formatter called ISO_INSTANT. As the name implies, this formatter provides a convenient way to format or parse an instant in UTC.

According to the official documentation, an instant cannot be formatted as a date or time without specifying a time zone. So, attempting to use ISO_INSTANT on LocalDateTime or LocalDate objects will lead to an exception:

@Test(expected = UnsupportedTemporalTypeException.class)
public void shouldExpectAnExceptionIfInputIsLocalDateTime() {
    DateTimeFormatter.ISO_INSTANT.format(LocalDateTime.now());
}

However, we can use ISO_INSTANT to format a ZonedDateTime instance without any issue:

@Test
public void shouldPrintFormattedZonedDateTime() {
    ZonedDateTime zonedDateTime = ZonedDateTime.of(2021, 02, 15, 0, 0, 0, 0, ZoneId.of("Europe/Paris"));
    String formattedZonedDateTime = DateTimeFormatter.ISO_INSTANT.format(zonedDateTime);
    
    Assert.assertEquals("2021-02-14T23:00:00Z", formattedZonedDateTime);
}

As we can see, we created our ZonedDateTime with “Europe/Paris” time zone. However, the formatted result is in UTC.

Similarly, when parsing to ZonedDateTime, we need to specify the time zone:

@Test
public void shouldParseZonedDateTime() {
    DateTimeFormatter formatter = DateTimeFormatter.ISO_INSTANT.withZone(ZoneId.systemDefault());
    ZonedDateTime zonedDateTime = ZonedDateTime.parse("2021-10-01T05:06:20Z", formatter);
    
    Assert.assertEquals("2021-10-01T05:06:20Z", DateTimeFormatter.ISO_INSTANT.format(zonedDateTime));
}

Failing to do so will lead to DateTimeParseException:

@Test(expected = DateTimeParseException.class)
public void shouldExpectAnExceptionIfTimeZoneIsMissing() {
    ZonedDateTime zonedDateTime = ZonedDateTime.parse("2021-11-01T05:06:20Z", DateTimeFormatter.ISO_INSTANT);
}

It's also worth mentioning that parsing requires specifying at least the seconds field. Otherwise, DateTimeParseException will be thrown.

5. Conclusion

In this article, we discussed how to use the DateTimeFormatter class to format dates and times. We also examined real-life example patterns that often arise when we work with date-time instances.

We can find out more about Java 8's Date/Time API in previous tutorials. As always, the source code used in this article is available over on GitHub.

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