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I have a String containing a time in the format: 08:00:00

This time is from US Eastern time and I want to convert it to London's timezone and end up with a String of that time.

I have converted the String to time using

Time.valueOf(t);

However after this I cannot get the timezone to change.

Rpp
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  • You might want to show what you've tried so far (beyond that single line you posted). – Thomas Jul 12 '17 at 12:14
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    I am not sure what you are asking for. "08:00:00" in US eastern, and now **compute** the according time in London? That is **impossible** without a *Date* for example. In that sense: provide a [mcve] and clearly list input and expected output. – GhostCat Jul 12 '17 at 12:14

2 Answers2

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you can displace the time using withZoneSameInstant

LocalTime myLocalTime = LocalTime.parse("08:00:00", DateTimeFormatter.ofPattern("HH:mm:ss"));
LocalTime londonTime = LocalDateTime.of(LocalDate.now(), myLocalTime).atZone(ZoneId.of("America/New_York"))
        .withZoneSameInstant(ZoneId.of("Europe/London")).toLocalTime();

System.out.println(myLocalTime);
System.out.println(londonTime);
ΦXocę 웃 Пepeúpa ツ
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2

There are lots of details regarding this question.

The Time class sets the date (day, month and year) to January 1st, 1970. But to convert from EST to London local time, you must consider Daylight Saving Time rules.
The difference in hours is not always the same; it can change depending on the date - considering this year (2017): from January 1st to March 11th, the difference will be 5 hours, then from March 12th to March 25th the difference is 4 hours, then it's back to 5 hours, then in October 29th it's 4 hours and in November 5th is 5 hours again, until the end of the year.

That's because of DST starting and ending in both timezones and at different dates. And each year, these dates change as well, so you need to know the date you're working with, to make the correct conversion.

Another thing is that Java 8 new API uses IANA timezones names (always in the format Region/City, like America/Sao_Paulo or Europe/Berlin). Avoid using the 3-letter abbreviations (like CST or EST) because they are ambiguous and not standard.

If you're using Java <= 7, you can use the ThreeTen Backport, a great backport for Java 8's new date/time classes. And for Android, there's the ThreeTenABP (more on how to use it here).

The code below works for both. The only difference is the package names (in Java 8 is java.time and in ThreeTen Backport (or Android's ThreeTenABP) is org.threeten.bp), but the classes and methods names are the same.

In the example below I'm using America/New_York - one of the many timezones that uses EST (there are more than 30 timezones that uses or had used it). You can call ZoneId.getAvailableZoneIds() to check all the timezones and choose one that suits best for your case.

The code is very similar to @ΦXocę 웃 Пepeúpa ツ answer, well, because it's straightforward and there's not much to change. I just wanted to add the insights above.

// timezones for US and UK
ZoneId us = ZoneId.of("America/New_York");
ZoneId uk = ZoneId.of("Europe/London");
// parse the time string
LocalTime localTimeUS = LocalTime.parse("08:00:00");
// the reference date (now is the current date)
LocalDate now = LocalDate.now(); // or LocalDate.of(2017, 5, 20) or any date you want
// the date and time in US timezone
ZonedDateTime usDateTime = ZonedDateTime.of(now, localTimeUS, us);
// converting to UK timezone
ZonedDateTime ukDateTime = usDateTime.withZoneSameInstant(uk);
// get UK local time
LocalTime localTimeUK = ukDateTime.toLocalTime();
System.out.println(localTimeUK);

The output will be 13:00 (the result of localTimeUK.toString()) because toString() omits the seconds if the value is zero.

If you want to always output the seconds, you can use a DateTimeFormatter:

DateTimeFormatter fmt = DateTimeFormatter.ofPattern("HH:mm:ss");
String time = fmt.format(localTimeUK);

In this case, the string time will be 13:00:00.

LocalDate.now() returns the current date using your system's default timezone. If you want the current date in a specific zone, you could've called LocalDate.now(us) (or anyzone you want, or even explicit use the default: LocalDate.now(ZoneId.systemDefault()))