datetime - How to change TIMEZONE for a java.util.Calendar/Date

Question

I would like to change the TIMEZONE value in a Java Calendar instance at runtime. I tried below. But the output is the same in both instances:

    Calendar cSchedStartCal = Calendar.getInstance(TimeZone.getTimeZone("GMT"));
    System.out.println(cSchedStartCal.getTime().getTime());
    cSchedStartCal.setTimeZone(TimeZone.getTimeZone("Asia/Calcutta"));
    System.out.println(cSchedStartCal.getTime().getTime());

OUTPUT:
1353402486773
1353402486773

I have tried this also but the output is still the same:

    Calendar cSchedStartCal = Calendar.getInstance(TimeZone.getTimeZone("GMT"));
    System.out.println(cSchedStartCal.getTime());

    Calendar cSchedStartCal1 = Calendar.getInstance(TimeZone.getTimeZone("Asia/Calcutta"));
    cSchedStartCal1.setTime(cSchedStartCal.getTime());
    System.out.println(cSchedStartCal.getTime());

In the API I am seeing the below comment but I am not able to understand much of it:

     * calls: cal.setTimeZone(EST); cal.set(HOUR, 1); cal.setTimeZone(PST).
     * Is cal set to 1 o'clock EST or 1 o'clock PST?  Answer: PST.  More
     * generally, a call to setTimeZone() affects calls to set() BEFORE AND
     * AFTER it up to the next call to complete().

Could you please help me?

One Possible Solution :

    Calendar cSchedStartCal = Calendar.getInstance(TimeZone.getTimeZone("GMT"));
    long gmtTime = cSchedStartCal.getTime().getTime();

    long timezoneAlteredTime = gmtTime + TimeZone.getTimeZone("Asia/Calcutta").getRawOffset();
    Calendar cSchedStartCal1 = Calendar.getInstance(TimeZone.getTimeZone("Asia/Calcutta"));
    cSchedStartCal1.setTimeInMillis(timezoneAlteredTime);

Is this solution ok?

Answer 1

In Java, Dates are internally represented in UTC milliseconds since the epoch (so timezones are not taken into account, that's why you get the same results, as getTime() gives you the mentioned milliseconds).
In your solution:

Calendar cSchedStartCal = Calendar.getInstance(TimeZone.getTimeZone("GMT"));
long gmtTime = cSchedStartCal.getTime().getTime();

long timezoneAlteredTime = gmtTime + TimeZone.getTimeZone("Asia/Calcutta").getRawOffset();
Calendar cSchedStartCal1 = Calendar.getInstance(TimeZone.getTimeZone("Asia/Calcutta"));
cSchedStartCal1.setTimeInMillis(timezoneAlteredTime);

you just add the offset from GMT to the specified timezone ("Asia/Calcutta" in your example) in milliseconds, so this should work fine.

Another possible solution would be to utilise the static fields of the Calendar class:

//instantiates a calendar using the current time in the specified timezone
Calendar cSchedStartCal = Calendar.getInstance(TimeZone.getTimeZone("GMT"));
//change the timezone
cSchedStartCal.setTimeZone(TimeZone.getTimeZone("Asia/Calcutta"));
//get the current hour of the day in the new timezone
cSchedStartCal.get(Calendar.HOUR_OF_DAY);

Refer to stackoverflow.com/questions/7695859/ for a more in-depth explanation.