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1. Overview

In this tutorial, we’ll review the Java 8 DateTimeFormatter class and its formatting patterns. We’re also going to 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 that we receive as a String of a known format. 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’ll have to use 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));

Notice that we have used ofLocalizedDateTime() method of DateTimeFormatter this time.

And 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 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 will 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 will get a “full form” representation. In our example, it is “July”. A 5-letter pattern, “MMMMM”, will make the formatter use the “narrow form”. In our case, “J” would be used.

Likewise, custom formatting pattern 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, and 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’d like 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));

Which gives 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 time String with our custom formatter and check if it is 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 happen to use New York-based date-time (UTC -4), we may 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 will get a DateTimeParseException.

5. Conclusion

In this tutorial, we’ve discussed how to use the DateTimeFormatter class for format dates and times. We’ve used real-life example patterns that often arise when we work with date-time instances.

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

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