Joda Time gives wrong time zone

Question

I'm using the Joda time (1.6) libraries and it keeps returning DateTime objects with the wrong time zone, British Summer Time instead of GMT.

My Windows workstation (running JDK 1.6.0_16) thinks it's in GMT and if I get the default time zone from the JDK date/time classes it is correct (GMT). I get the same behaviour on our Linux servers as well. I thought it could be an error in the time zone database files in Joda so I rebuilt the jar with the latest database but with no change.

import java.util.TimeZone;
import org.joda.time.DateTime;
import org.joda.time.DateTimeZone;
import org.joda.time.LocalTime;
import org.joda.time.format.DateTimeFormatter;
import org.joda.time.format.ISODateTimeFormat;

public class TimeZoneTest {

    public static void main(String[] args) {                
        DateTimeFormatter timeParser = ISODateTimeFormat.timeParser();
        TimeZone timeZone = TimeZone.getDefault();
        System.out.println(timeZone.getID()); // "Europe/London"
        System.out.println(timeZone.getDisplayName()); // "Greenwich Mean Time"

        DateTimeZone defaultTimeZone = DateTimeZone.getDefault();
        System.out.println(defaultTimeZone.getID()); //"Europe/London"
        System.out.println(defaultTimeZone.getName(0L)); //"British Summer Time"

        DateTime currentTime = new DateTime();
        DateTimeZone currentZone = currentTime.getZone();
        System.out.println(currentZone.getID()); //"Europe/London"
        System.out.println(currentZone.getName(0L)); //"British Summer Time"            
    }
}

Debugging through the static initialiser in org.joda.time.DateTimeZone I see that the System.getProperty("user.timezone") call gives "Europe/London" as expected.

Answer 1

Ok, to get to the root of this you have to familiarize with what British Summer Time actually means and when it was in place. To make it short you pass 0L to getName() which is 1970-01-01T00:00:00Z so DefaultTimeZone looks up the name of the timezone at that moment. Which was British Summer Time.

From: http://www.nmm.ac.uk/explore/astronomy-and-time/time-facts/british-summer-time

In 1968 clocks were advanced one hour ahead of GMT on 18 February and remained so until British Standard Time, during which clocks were kept in advance of GMT all year, came into force between 27 October 1968 and 31 October 1971.

If you instead would pass in the right amount of milliseconds since 1970-01-01T00:00:00Z. e.g. by doing

defaultTimeZone.getName(new GregorianCalendar().getTimeInMillis())

You would get the correct string too. Basically you just fed the getName() method the wrong parameter thus ended up with an unexpected result.

If you want to check in detail check the files in org/joda/time/tz/src of joda source to see how joda determines time zones.

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Instead of

defaultTimeZone.getName(0L)

you could use

defaultTimeZone.toTimeZone().getDisplayName()

which does it for me.