Strange problem with timezone, calendar and Simple

问题:

Let's consider the following code:

SimpleDateFormat sdf = new SimpleDateFormat("HH:mm:ss dd/MM/yyyy", Locale.US);
long start = sdf.parse("10:30:00 30/09/2009").getTime();
long end = sdf.parse("10:30:00 30/10/2009").getTime();

Calendar c = Calendar.getInstance(Locale.US);
c.setTimeInMillis(start);
System.out.println("Start = " + c.getTime());
c.setTimeInMillis(end);
System.out.println("  End = " + c.getTime());

When running this code snippet, I have the following output:

Start = Wed Sep 30 10:30:00 CEST 2009
  End = Fri Oct 30 10:30:00 CET 2009

Why do I get different timezone ?

Note that if I set the first date in august and the second one in september, the output will display the same timezone in both cases:

long start = sdf.parse("10:30:00 30/08/2009").getTime();
long end = sdf.parse("10:30:00 30/09/2009").getTime();

will display:

Start = Sun Aug 30 10:30:00 CEST 2009
  End = Wed Sep 30 10:30:00 CEST 2009

I'm using Java 1.6.0_14

回答1:

CEST is Central European Summer Time. It is the same as CET with daylight savings into effect.

回答2:

You can set the default time zone

    import java.util.TimeZone;
...        
    TimeZone.setDefault(TimeZone.getTimeZone("UTC"));  // or "Etc/GMT-1"

    SimpleDateFormat sdf = new SimpleDateFormat("HH:mm:ss dd/MM/yyyy", Locale.US);
    long start = sdf.parse("10:30:00 30/09/2009").getTime();
    long end = sdf.parse("10:30:00 30/10/2009").getTime();

    Calendar c = Calendar.getInstance(Locale.US);
    c.setTimeInMillis(start);
    System.out.println("Start = " + c.getTime());
    c.setTimeInMillis(end);
    System.out.println("  End = " + c.getTime());

use TimeZone.getAvailableIDs() to see all available IDs.

EDIT: you can also use a new SimpleTimeZone

    TimeZone.setDefault(new SimpleTimeZone(60 * 60 * 1000, "CET"));

回答3:

Yes, it is related to the daylight saving time. If you use a time zone that recognizes DST, it will be automatically used. You can use GMT for example if you don't want this.