Get Daylight Saving Transition Dates For Time Zones in Java

Question

I\'d like to know the simplest way, in Java, to get a list of dates in the future where daylight savings time will change.

One rather inellegant way to do this would

Answer 1

java.time

The modern answer uses java.time, the modern Java date and time API.

    ZoneId zone = ZoneId.of("Europe/London");
    ZoneRules rules = zone.getRules();
    ZonedDateTime now = ZonedDateTime.now(zone);
    ZoneOffsetTransition transition = rules.nextTransition(now.toInstant());
    Instant max = now.plusYears(15).toInstant();
    while (transition != null && transition.getInstant().isBefore(max)) {
        System.out.println(transition);
        transition = rules.nextTransition(transition.getInstant());
    }

Output, abbreviated:

Transition[Overlap at 2019-10-27T02:00+01:00 to Z]
Transition[Gap at 2020-03-29T01:00Z to +01:00]
Transition[Overlap at 2020-10-25T02:00+01:00 to Z]
Transition[Gap at 2021-03-28T01:00Z to +01:00]
Transition[Overlap at 2021-10-31T02:00+01:00 to Z]
Transition[Gap at 2022-03-27T01:00Z to +01:00]
Transition[Overlap at 2022-10-30T02:00+01:00 to Z]
(cut)
Transition[Overlap at 2033-10-30T02:00+01:00 to Z]
Transition[Gap at 2034-03-26T01:00Z to +01:00]

I would not put too much trust in the data, though. I am not sure what happens with time in the UK after Brexit (and after EU may abandon summer time (DST) in 2021).

Link: Oracle tutorial: Date Time explaining how to use java.time.

Answer 2

Joda Time (as ever) makes this really easy due to the DateTimeZone.nextTransition method. For example:

import org.joda.time.*;
import org.joda.time.format.*;

public class Test
{    
    public static void main(String[] args)
    {
        DateTimeZone zone = DateTimeZone.forID("Europe/London");        
        DateTimeFormatter format = DateTimeFormat.mediumDateTime();

        long current = System.currentTimeMillis();
        for (int i=0; i < 100; i++)
        {
            long next = zone.nextTransition(current);
            if (current == next)
            {
                break;
            }
            System.out.println (format.print(next) + " Into DST? " 
                                + !zone.isStandardOffset(next));
            current = next;
        }
    }
}

Output:

25-Oct-2009 01:00:00 Into DST? false
28-Mar-2010 02:00:00 Into DST? true
31-Oct-2010 01:00:00 Into DST? false
27-Mar-2011 02:00:00 Into DST? true
30-Oct-2011 01:00:00 Into DST? false
25-Mar-2012 02:00:00 Into DST? true
28-Oct-2012 01:00:00 Into DST? false
31-Mar-2013 02:00:00 Into DST? true
27-Oct-2013 01:00:00 Into DST? false
30-Mar-2014 02:00:00 Into DST? true
26-Oct-2014 01:00:00 Into DST? false
29-Mar-2015 02:00:00 Into DST? true
25-Oct-2015 01:00:00 Into DST? false
...

With Java 8, you can get the same information using ZoneRules with its nextTransition and previousTransition methods.