How does GMT IDs work in Java TimeZone?

StackOverflow https://stackoverflow.com/questions/23640227

  •  21-07-2023
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Question

I'm currently working with different time zones, using java.util.TimeZone. First thing I did was to get a list of TimeZone IDs, using TimeZone.getAvailableIDs();

It returned a bunch of Etc/GMT IDs, such as

Etc/GMT
Etc/GMT+0
Etc/GMT+1

The Javadoc for TimeZone says to use "For example, GMT+10 and GMT+0010"

To make sure I was using the correct one, I made a small test with both IDs, and the results didn't match what I was expecting:

String id = "GMT-1";
Calendar c = Calendar.getInstance(TimeZone.getTimeZone(id));
System.out.println(id + '\n' + c.getTimeZone().getDisplayName() + " HOUR_OF_DAY " + c.get(Calendar.HOUR_OF_DAY) + '\n');

id = "GMT+1";
c = Calendar.getInstance(TimeZone.getTimeZone(id));
System.out.println(id + '\n' + c.getTimeZone().getDisplayName() + " HOUR_OF_DAY " + c.get(Calendar.HOUR_OF_DAY) + '\n');

id = "Etc/GMT-1";
c = Calendar.getInstance(TimeZone.getTimeZone(id));
System.out.println(id + '\n' + c.getTimeZone().getDisplayName() + " HOUR_OF_DAY " + c.get(Calendar.HOUR_OF_DAY) + '\n');

id = "Etc/GMT+1";
c = Calendar.getInstance(TimeZone.getTimeZone(id));
System.out.println(id + '\n' + c.getTimeZone().getDisplayName() + " HOUR_OF_DAY " + c.get(Calendar.HOUR_OF_DAY) + '\n');

Shows me the output

GMT-1
GMT-01:00 HOUR_OF_DAY 18

GMT+1
GMT+01:00 HOUR_OF_DAY 20

Etc/GMT-1
GMT+01:00 HOUR_OF_DAY 20

Etc/GMT+1
GMT-01:00 HOUR_OF_DAY 18

So my question is: What are these Etc/GMT IDs for? Why would they give me -X when I used +X and vice versa?

Was it helpful?

Solution

The Etc/GMT... time zones are there primarily for backwards compatibility with POSIX standards, which specified positive offsets west of GMT instead of the typical convention east of GMT that you might be used to.

In most cases, you should not use them. For example, if you are in the US Pacific time zone, you should use the identifier America/Los_Angeles. It alternates between UTC-8 for standard time in the winter, and UTC-7 for daylight time in the summer. You shouldn't just use Etc/GMT+8, because that wouldn't account for daylight saving time.

Other than backwards compatibility, another area where these fixed-offset zones come into play is for nautical purposes. Ships at sea have a lot of control over how they set their time, but they typically use fixed offsets separated by 15 degrees of longitude. It would be perfectly acceptable for a ship in the Atlantic ocean to use a time zone setting identifier of Etc/GMT+3.

Just don't forget to invert the sign. Etc/GMT+3 = UTC-3

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