Dean kirjoitti:

Reinvent the wheel, or use Joda-time library :
<http://joda-time.sourceforge.net/> and you really don't have to care
about DST in your calculations.

--

Jani Tiainen

You should not need to do this for yourself. Calendar and GregorianCalendar
handle timezones and DST correctly if your system is setup properly.

Here in the UK DST came in at 2am on 25th March (the clocks went forward). This
code creates 4 GregorianCalendars, at 10pm on the 24th, at 01:59:59 and
02:00:01 and 03:00:00 on the 25th. The difference between them shows you that
the calendars are correct:

public class TestTime {
   public static void main(String[] args) {
       GregorianCalendar cal1 = new GregorianCalendar(2007,2,24,22,0,0);
       GregorianCalendar cal2 = new GregorianCalendar(2007,2,25,1,59,59);
       GregorianCalendar cal3 = new GregorianCalendar(2007,2,25,2,0,1);
       GregorianCalendar cal4 = new GregorianCalendar(2007,2,25,3,0,0);
       DateFormat fmt = SimpleDateFormat.getInstance();
       
       System.out.println(fmt.format(cal1.getTime()));
       System.out.println(fmt.format(cal2.getTime()));
       System.out.println(fmt.format(cal3.getTime()));
       System.out.println(fmt.format(cal4.getTime()));
       
       System.out.println((cal2.getTimeInMillis() -
cal1.getTimeInMillis())/60.0/60.0/1000.0);
       System.out.println((cal3.getTimeInMillis() -
cal1.getTimeInMillis())/60.0/60.0/1000.0);
       System.out.println((cal4.getTimeInMillis() -
cal1.getTimeInMillis())/60.0/60.0/1000.0);

   }
   
}

Run this code in the a UK timezone and you should get a result of 3.9997, 3.0002
and 4.0002. So 01:59:59 is almost 4 hours after 10pm, whereas 02:00:01 is only
3 hours after 10pm and 03:00:01 is very slightly over 4 hours after i.e
immediately following 01:59:59. If you do something similar for your
timezone/DST changeover you should get similar results.

I think these time differences are what you need to calculate how long a car was
parked during the switch in DST, provided the clocks on the parking meters
record time in the correct way.

see http://mindprod.com/jgloss/timezone.html
--
Roedy Green Canadian Mind Products
The Java Glossary
http://mindprod.com

The easiest way to handle time in Java (and in most languages) is simply to
use system time in milliseconds. It is a scalar value with no time zone or
daylight time discontinuities and no modulo arithmetic. You simply add or
subtract time1 to or from time2. Using a signed long, the system time range
is plus or minus about 292 million years from January 1, 1970, 00:00:00.00
GMT.

If you use system time in milliseconds for all of your internal timekeeping
calculations, when you convert it to or from a displayable format using
java.text.SimpleDateFormat, or java.util.Calendar, they will do the
conversion to the proper time zone and daylight time automatically as long
as the locale and the system clock are set correctly. You never even have to
think about it.

If you keep time as a long milliseconds since 1970 GST, you can simply
subtract to get intervals. It works across time zones, during DST
changes etc. without any special fiddles.

Your problem then becomes converting from human local time to the long
and back.  See http://mindprod.com/jgloss/calendar.html

http://mindprod.com/jgloss/time.html
--
Roedy Green Canadian Mind Products
The Java Glossary
http://mindprod.com