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I have an instance of the Date class and a time zone (e.g Europe/London). How do I convert this into an Instant? Is the below the correct way of doing this?

LocalDateTime localDateTime = LocalDateTime.ofInstant(date.toInstant(), ZoneId.systemDefault());
ZonedDateTime zonedDateTime = ZonedDateTime.of(localDateTime, ZoneId.of(timeZone));
Instant instant = zonedDateTime.toInstant();
Ole V.V.
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kcon123
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    what is an "UTC Instant"? An `Instant` is a "*An instantaneous point on the time-line.*", it does not *have* time zone information. (what about `Instant instant = date.toInstant()`?) – user16320675 May 25 '23 at 07:26
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    You don't need a `LocalDateTime`, there's `ZonedDateTime.ofInstant(Instant, ZoneId)` where you can put `date.toInstant()` and `ZoneOffset.UTC`. But if you want just a `String`, simply print the `Instant`. – deHaar May 25 '23 at 07:29
  • I've updated that, I just need an Instant. – kcon123 May 25 '23 at 07:29
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    `Date.toInstant()`… that's it. It is straight-forward because a `java.util.Date` is not much more than a wrapper around a value representing epoch millis, which is the base of an `Instant`, too. – deHaar May 25 '23 at 07:30
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    BTW that code should not compile, assuming `java.util.Date` or subclasses ("*I have an instance of the Date class*"), because `LocalDateTime.ofInstant()` *requires* an `Instant` (answering "*Is the below the correct way of doing this?*") || BTW2 check the [documentation](https://docs.oracle.com/en/java/javase/20/docs/api/java.base/java/util/Date.html) to find all available methods) – user16320675 May 25 '23 at 07:32
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    Which `Date` class did you mean? – Basil Bourque May 25 '23 at 07:34
  • Trying a guess: Are you having a `java.util.Date` that when you print it *appears* to have the date and time that you want in the wanted time zone? For example `Thu May 25 11:44:35 CEST 2023` where you want an `Instant` representing the same moment as `2023-05-25T11:44:35+01:00[Europe/London]`? In this case the `Date` is an incorrect representation of that moment (instant). Such incorrect representations are sometimes seen exactly because `Date` is so confusing. – Ole V.V. May 25 '23 at 09:58
  • In case my guess is spot-on, your code does make the conversion you asked for. – Ole V.V. May 25 '23 at 11:05

1 Answers1

4

tl;dr

myJavaUtilDate.toInstant()

No need for LocalDateTime, ZoneId, ZonedDateTime classes.

Details

You said:

I have an instance of the Date class

java.util.Date myJavaUtilDate = … ;

This class is now legacy, years ago supplanted by the modern java.time classes defined in JSR 310. Specifically replaced by java.time.Instant, to represent a moment, a specific point on the timeline, as seen with an offset of zero hours-minutes-seconds from the temporal meridian of UTC.

and a time zone (e.g Europe/London)

ZoneId z = ZoneId.of( "Europe/London" ) ;

How do I convert this into an Instant?

No need for the time zone. Both java.util.Date and java.time.Instant represent a moment as seen in UTC, as described above.

New conversion methods were added to the old classes. Look for the naming conventions of to… & from…. You can convert back and forth between the legacy classes and the modern classes. Stick to the modern classes. Use the legacy date-time classes only to interoperate with old code not yet updated to java.time.

java.time.Instant instant = myJavaUtilDate.toInstant() ;

And the other direction:

java.util.Date d = java.util.Date.from( instant ) ;

Beware of data loss in this other direction. An Instant has a resolution of nanoseconds, while java.util.Date resolves to milliseconds.

Basil Bourque
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