Question

My requirment is like this:

I am saving time in millisecond in database and the timezone.For example the time in milisecond is 1223123123232 in long and timezone is Asia/Calcutta. I have to convert it to Africa/Asmara timezone.

long l = 1223123123232l;
TimeZone tz = TimeZone.getTimeZone("Asia/Calcutta");
long tzOff = tz.getOffset(l);
java.util.Date d = new Date(l-tzOff);   // WHY THIS??
DateFormat df = new SimpleDateFormat("dd MMM yyyy HH:mm:ss z");
df.setTimeZone(TimeZone.getTimeZone("Africa/Asmara"));// required timezone
String s = df.format(d);
System.out.println(s);

To check i am refering this: link

My question is:

1. If the timezone is just the representation of time in different formats(geographical areas offset from GMT),why do i need to subtract the offset time form the actual time (l-tzOff)?
2. Why can’t i ignore the timezone which is saved in Database, and only consider the timezome in which i want to convert the date?

Something like:

long l = 1223123123232l;
java.util.Date d = new Date(l);
DateFormat df = new SimpleDateFormat("dd MMM yyyy HH:mm:ss z");
df.setTimeZone(TimeZone.getTimeZone("Africa/Asmara"));// required timezone
String s = df.format(d);
System.out.println(s);

My system timezone is Asia/Calcutta, i want to convert a Date in Africa/Bujumbura timezone to Europe/Vatican timezone.The above code is not working in this case? Why this is so?

Answer 1

Java dates do not know about time zones. Therefore, if I save 14H00 in New-York, it will not be the same 14H00 as in Paris, although the millisecond value is the same. You need to use a unique reference to save dates. People often chose GMT+0.

If you need to check local time across timezones, you can use a tool I developed here.

To answer your questions:

1. Timezone is not a representation of time, it is a localisation of time. The reason you would substract an offset is to make sure all time is defined according to the same reference.
2. Because you would get false results and false dates.