Algorithm to determine Daylight Saving Time of a date?

Question

Originally I am looking for a solution in Actionscript. The point of this question is the algorithm, which detects the exact Minute, when a clock has to switch the Daylight Saving Time.

So for example between the 25th and the 31th of October we have to check, if the actual date is a sunday, it is before or after 2 o'clock...

Answer 1

perhaps this will help? http://dir.filewatcher.com/d/Ubuntu/all/libs/tzdata-java_2008b-1ubuntu1_all.deb.139946.html

Answer 2

There is no real algorithm for dealing with Daylight Saving Time. Basically every country can decide for themselves when -and if- DST starts and ends. The only thing we can do as developers is using some sort of table to look it up. Most computer languages integrate such a table in the language.

In Java you could use the inDaylightTime method of the TimeZone class. If you want to know the exact date and time when DST starts or ends in a certain year, I would recommend to use Joda Time. I can't see a clean way of finding this out using just the standard libraries.

The following program is an example: (Note that it could give unexpected results if a certain time zone does not have DST for a certain year)

import org.joda.time.DateTime;
import org.joda.time.DateTimeZone;

public class App {
    public static void main(String[] args) {
        DateTimeZone dtz = DateTimeZone.forID("Europe/Amsterdam");

        System.out.println(startDST(dtz, 2008));
        System.out.println(endDST(dtz, 2008));
    }

    public static DateTime startDST(DateTimeZone zone, int year) {
        return new DateTime(zone.nextTransition(new DateTime(year, 1, 1, 0, 0, 0, 0, zone).getMillis()));
    }

    public static DateTime endDST(DateTimeZone zone, int year) {
        return new DateTime(zone.previousTransition(new DateTime(year + 1, 1, 1, 0, 0, 0, 0, zone).getMillis()));
    }
}