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_GBinstant: 2015-06-02T21:34:33.616Zoutput: 02/06/15 14:34

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()));}

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))