[java] Format Instant to String

I'm trying to format an Instant to a String using the new java 8 time-api and a 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 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)
    ...

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)

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))

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

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.

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()));

}

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");