Convert a GMT datetime to local timezone datetime

Question

In Java 8 I need a way to get the local datetime (GMT+1) from a GMT datetime in ISO 8601 format.

A simple example: Client sends me (Server) this datetime "2020-01-11T23:00:00.000Z" Client sends me this when the user choose the 12 Jan 2020 from the datepicker. Is the 12 Jan for GMT+1 but the day before for GMT.

For the reason above then I know that for me this datetime is not the 11 Jan 2020 but 12 Jan 2020 in GMT+1.

So I need this value "2020-01-12T00:00:00.000"

To be precise I don't need to print this with simpleDateFormat but just covert "2020-01-11T23:00:00.000Z" to "2020-01-12T00:00:00.000" in a java.util.Date class field

Thanks.

Answer 1

The problem is that the source system took the pure date value, but added time at midnight, then converted that to UTC, but you want the pure date value in a java.util.Date, which by default prints in your local time zone, i.e. the JVM's default time zone.

So, you have to parse the string, revert the value back to the time zone of the source system, the treat that local time as a time in your own JVM's default time zone.

You can do that like this, showing all the intermediate types:

String sourceStr = "2020-01-11T23:00:00.000Z";
ZoneId sourceTimeZone = ZoneOffset.ofHours(1); // Use real zone of source, e.g. ZoneId.of("Europe/Paris");

// Parse Zulu date string as zoned date/time in source time zone
Instant sourceInstant = Instant.parse(sourceStr);
ZonedDateTime sourceZoned = sourceInstant.atZone(sourceTimeZone);

// Convert to util.Date in local time zone
ZonedDateTime localZoned = sourceZoned.withZoneSameLocal(ZoneId.systemDefault());
Instant localInstant = localZoned.toInstant();
Date localDate = Date.from(localInstant); // <== This is your desired result

// Print value in ISO 8601 format
String localStr = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS").format(localDate);
System.out.println(localStr);

Output

2020-01-12T00:00:00.000

---

The code can of course be merged together:

String input = "2020-01-11T23:00:00.000Z";

Date date = Date.from(Instant.parse(input).atZone(ZoneOffset.ofHours(1))
        .withZoneSameLocal(ZoneId.systemDefault()).toInstant());

System.out.println(date);

Output

Sun Jan 12 00:00:00 EST 2020

As you can see, the date value is correct, even though I'm in the US Eastern time zone.