Day Light Saving Offset when to add/substract

[Rohit Shaarma]

Hi,

I have a problem with day light saving time in java, i am able to get time zone offset (i.e, the amount of time by which the clock is forwarded or reversed) but i am not able to get the condition when the clock is forwarded/reversed.

Any help is greatly appreciated.


Thanks in advance
Rohit Sharma

[Jesper de Jong]

Welcome to the Ranch.

So, you want to know for a specific timezone what the rules are for switching from summertime to wintertime and vice versa?

The standard java.util.TimeZone class does not provide that information. In fact, the rules can be quite complicated and differ from year to year - governments around the world are changing timezone rules all the time.

The Joda Time library might help. It has a DateTimeZone class, which as far as I can see also doesn't tell you the exact rules, but it does have methods to get when the next or previous transition was to or from summertime or wintertime.

[Rohit Shaarma]

Thank you Jesper, basically my problem is i have a scheduler which runs every day at 12:15PM and due to DST it is running an hour before i.e, at 11:15PM and since as you said TimeZone doesnt provide any rule it is just providing an usigned offset of 1 hour(3600000 msec) what would be the easiest way to handle this issue according to you.




Regards,
Rohit Sharma

[Rohit Shaarma]

I found a simple solution. I just checked whether either current time or scheduled time were in DST, based on that i added/substracted the DST offset. below is a sample code snippet

TimeZone tz1 = TimeZone.getDefault();
	  Calendar cal1 = Calendar.getInstance(tz1);
	  cal1.setTimeInMillis(nextResetTime);
	  if(tz1.inDaylightTime(new Date(System.currentTimeMillis())) && !tz1.inDaylightTime(cal1.getTime())){
		  if (debug) _log.debug("Adding an offset of "+tz1.getDSTSavings()+" msecs");
		  cal1.add(Calendar.MILLISECOND,tz1.getDSTSavings());
	  }else if(!tz1.inDaylightTime(new Date(System.currentTimeMillis())) && tz1.inDaylightTime(cal1.getTime())){
		  if (debug) _log.debug("Substracting an offset of "+tz1.getDSTSavings()+" msecs");
		  cal1.add(Calendar.MILLISECOND,-tz1.getDSTSavings());
	  }
	  if (debug) _log.debug("Next reset time after DST calculation : "+new Date(cal1.getTimeInMillis()).toString());
	  _nextRunTime = cal1.getTimeInMillis();

Regards,
Rohit Sharma

[Rob Spoor]

Instead of new Date(System.currentTimeMillis()) you can also use new Date(); they are the same thing.

[Winston Gutkowski]

Rohit Shaarma wrote:Thank you Jesper, basically my problem is i have a scheduler which runs every day at 12:15PM and due to DST it is running an hour before i.e, at 11:15PM and since as you said TimeZone doesnt provide any rule it is just providing an usigned offset of 1 hour(3600000 msec) what would be the easiest way to handle this issue according to you.

Have your scheduler determine the start time based on java.util.Calendar, rather than java.util.Date. The problem isn't with TimeZone, but rather, I suspect, the fact that your scheduler isn't taking "clock time" into account. And I hate to say it, but your nifty piece of code looks suspiciously like a solution to the symptom, not the problem.

Winston