CODEWITHSUNDEEP
timejava-8formattingtimezone-offset
Simeon Leyzerzon
Simeon Leyzerzon

Reputation: 19084

Instant differs from the Timestamp in Java?

I seem to be getting different representations of Java's time stamp as follows:

en_US
2017-06-10 14:34:35.088
2017-06-10T18:34:35.102Z

with following invocations:

System.out.println(Locale.getDefault());
System.out.println(new Timestamp(System.currentTimeMillis()));
System.out.println(Instant.now());

What additional configurations will help to equalize the the 4 hr offset between time representations shown above? Thanks.

Upvotes: 1

Views: 12415

Answers (1)

Marvin
Marvin

Reputation: 14415

One is your local time and one is UTC. This depends more on the way it is printed than on the actual (internal) value.

To have both shown equal, you can

  • Set your local time zone to UTC by passing -Duser.timezone=UTC to your VM.

  • Format the Instant to your local time zone:

    DateTimeFormatter formatter = 
DateTimeFormatter.ISO_LOCAL_DATE_TIME.withZone(ZoneId.systemDefault());
System.out.println(formatter.format(Instant.now()));

  • Convert the Timestamp to an Instant:

    new Timestamp(System.currentTimeMillis()).toInstant();

Full code:

public static void main(String[] args) {
    DateTimeFormatter formatter = DateTimeFormatter.ISO_LOCAL_DATE_TIME.withZone(ZoneId.systemDefault());
    Timestamp timestamp = new Timestamp(System.currentTimeMillis());
    Instant instant = Instant.now();
    System.out.println("Timestamp: " + timestamp);
    System.out.println("Instant: " + instant);
    System.out.println("Instant to local: " + formatter.format(instant));
    System.out.println("Timestamp to UTC: " + timestamp.toInstant());
}

Will print out (in my local time zone):

Timestamp: 2017-06-10 21:17:13.935
Instant: 2017-06-10T19:17:13.935Z
Instant to local: 2017-06-10T21:17:13.935
Timestamp to UTC: 2017-06-10T19:17:13.935Z

So there is "some global setting", although it might not do what you want. An Instant will always be printed in UTC by default.

If you want to work with local dates only and you are not restricted to Instant, there are two* more options:

  • Convert the Instant to ZonedDateTime:

    ZonedDateTime.ofInstant(Instant.now(), ZoneId.systemDefault());

  • Or skip the Instant altogether in favor of ZonedDateTime.now()

*Depending on your needs you can exchange ZonedDateTime with OffsetDateTime or LocalDateTime.

Edit: The magical way.

public static void main(String[] args) throws Exception {
    Field field = DateTimeFormatter.class.getField("ISO_INSTANT");
    field.setAccessible(true);
    Field modifiersField = Field.class.getDeclaredField("modifiers");
    modifiersField.setAccessible(true);
    modifiersField.setInt(field, field.getModifiers() & ~Modifier.FINAL);
    field.set(null, DateTimeFormatter.ISO_LOCAL_DATE_TIME.withZone(ZoneId.systemDefault()));

    System.out.println(new Timestamp(System.currentTimeMillis()));
    System.out.println(Instant.now());
}

Use reflection to change DateTimeFormatter.ISO_INSTANT to a formatter that uses your local time. Might not work with a SecurityManager and might have one or another unwanted side effect. See e.g. Change private static final field using Java reflection for details.

Upvotes: 2

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