I'm trying to format an Instant to a String using the new Java 8 Date and Time API and the following pattern:

Instant instant = ...; String out = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss").format(instant);

Using the code above I get an exception which complains about an unsupported field:

java.time.temporal.UnsupportedTemporalTypeException: Unsupported field: YearOfEra at java.time.Instant.getLong(Instant.java:608) at java.time.format.DateTimePrintContext.getValue(DateTimePrintContext.java:298) ...
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7 Answers

Time Zone

To format an Instant a time-zone is required. Without a time-zone, the formatter does not know how to convert the instant to human date-time fields, and therefore throws an exception.

The time-zone can be added directly to the formatter using withZone().

DateTimeFormatter formatter = DateTimeFormatter.ofLocalizedDateTime( FormatStyle.SHORT ) .withLocale( Locale.UK ) .withZone( ZoneId.systemDefault() );

If you specifically want an ISO-8601 format with no explicit time-zone (as the OP asked), with the time-zone implicitly UTC, you need

DateTimeFormatter.ISO_LOCAL_DATE_TIME.withZone(ZoneId.from(ZoneOffset.UTC))

Generating String

Now use that formatter to generate the String representation of your Instant.

Instant instant = Instant.now(); String output = formatter.format( instant );

Dump to console.

System.out.println("formatter: " + formatter + " with zone: " + formatter.getZone() + " and Locale: " + formatter.getLocale() ); System.out.println("instant: " + instant ); System.out.println("output: " + output );

When run.

formatter: Localized(SHORT,SHORT) with zone: US/Pacific and Locale: en_GB instant: 2015-06-02T21:34:33.616Z output: 02/06/15 14:34
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public static void main(String[] args) { DateTimeFormatter DATE_TIME_FORMATTER = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss") .withZone(ZoneId.systemDefault()); System.out.println(DATE_TIME_FORMATTER.format(new Date().toInstant())); }
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DateTimeFormatter.ISO_INSTANT.format(Instant.now())

This saves you from having to convert to UTC. However, some other language's time frameworks may not support the milliseconds so you should do 

DateTimeFormatter.ISO_INSTANT.format(Instant.now().truncatedTo(ChronoUnit.SECONDS))
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The Instant class doesn't contain Zone information, it only stores timestamp in milliseconds from UNIX epoch, i.e. 1 Jan 1070 from UTC. So, formatter can't print a date because date always printed for concrete time zone. You should set time zone to formatter and all will be fine, like this :

Instant instant = Instant.ofEpochMilli(92554380000L); DateTimeFormatter formatter = DateTimeFormatter.ofLocalizedDateTime(FormatStyle.SHORT).withLocale(Locale.UK).withZone(ZoneOffset.UTC); assert formatter.format(instant).equals("07/12/72 05:33"); assert instant.toString().equals("1972-12-07T05:33:00Z");
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Instants are already in UTC and already have a default date format of yyyy-MM-dd. If you're happy with that and don't want to mess with time zones or formatting, you could also toString() it:

Instant instant = Instant.now(); instant.toString() output: 2020-02-06T18:01:55.648475Z


Don't want the T and Z? (Z indicates this date is UTC. Z stands for "Zulu" aka "Zero hour offset" aka UTC):

instant.toString().replaceAll("[TZ]", " ") output: 2020-02-06 18:01:55.663763


Want milliseconds instead of nanoseconds? (So you can plop it into a sql query):

instant.truncatedTo(ChronoUnit.MILLIS).toString().replaceAll("[TZ]", " ") output: 2020-02-06 18:01:55.664

etc.

Or if you still want to use formatter created from pattern you can just use LocalDateTime instead of Instant:

LocalDateTime datetime = LocalDateTime.now(); DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss").format(datetime)
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DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy MM dd"); String text = date.toString(formatter); LocalDate date = LocalDate.parse(text, formatter);

I believe this might help, you may need to use some sort of localdate variation instead of instant

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