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javacalendartimezonedst

Get actual DST savings in milliseconds

I'm making analog clock in Java Swing. To calculate clock pointer angles I need:

  1. UTC time in milliseconds which I can get with System.currentTimeMillis()
  2. TimeZone offset
  3. TimeZone DST savings.

For 2) and 3) I thought of using TimeZone.getDefault().getRawOffset() and TimeZone.getDefault().getDSTSavings().

But getDSTSavings() always returns 3600000 regardless of current date is in winter/summer time.
It looks like it only checks to see if that zone is under DST management and if so it returns 3600000 otherwise 0.
Is this a bug in getDSTSavings()?
If not then only way to get current DST correction in miliseconds is to use Calendar instance like: Calendar.getInstance.get(Calendar.DST_OFFSET)? 

Calendar cal1 = Calendar.getInstance(TimeZone.getDefault());        //currentZone: CET/CEST +1/+2, GMT+1:00
System.out.println("System time, " + System.currentTimeMillis()); //UTC current milis
System.out.println("Calendar time, " + cal1.getTime().getTime());   //UTC current milis
System.out.println("Calendar milis, " + cal1.getTimeInMillis());       //UTC current milis
System.out.println("Calendar Zone Offset: " + cal1.get(Calendar.ZONE_OFFSET));
System.out.println("Calendar DST Offset: " + cal1.get(Calendar.DST_OFFSET));
System.out.println("Calendar Zone Offset: " + cal1.getTimeZone().getRawOffset());
System.out.println("Calendar DST Offset: " + cal1.getTimeZone().getDSTSavings());
System.out.println("");

// Winter time, CET
cal1.set(2010, 11, 15, 14, 15, 5);
System.out.println("Calendar milis, " + cal1.getTimeInMillis()); //UTC
System.out.println("Calendar Zone Offset: " + cal1.get(Calendar.ZONE_OFFSET)); // 3600000 correct
System.out.println("Calendar DST Offset: " + cal1.get(Calendar.DST_OFFSET)); // 0 correct
System.out.println("Calendar Zone Offset: " + cal1.getTimeZone().getRawOffset()); // 3600000 correct
System.out.println("Calendar DST Offset: " + cal1.getTimeZone().getDSTSavings()); // 3600000 wrong !!!
System.out.println("");

// Summer time - CEST
cal1.set(2010, 8, 15, 14, 15, 5);
System.out.println("Calendar milis, " + cal1.getTimeInMillis()); //UTC
System.out.println("Calendar Zone Offset: " + cal1.get(Calendar.ZONE_OFFSET)); // 3600000 correct
System.out.println("Calendar DST Offset: " + cal1.get(Calendar.DST_OFFSET)); // 3600000 correct
System.out.println("Calendar Zone Offset: " + cal1.getTimeZone().getRawOffset()); // 3600000 correct
System.out.println("Calendar DST Offset: " + cal1.getTimeZone().getDSTSavings()); // 3600000 correct
Solution

  • Here is an example of how DST is used:

    TimeZone london = TimeZone.getTimeZone("Europe/London");
System.out.println(london.getOffset(date.getTime()));

    This will print 3600000 if Daylight Saving Time is in effect in London at the specified date. Otherwise it will print 0.