Determine dates of DST switch (both forwards and backwards) for a timezone in Java?

Question

I realize that you can check if a Date is inDaylightTime, but how can I determine when (or if) that Date switches to and from DST? I can iterate over every day and check if it's inDaylightTime, but is there a cleaner solution/method?

Answer 1

I would suggest using Joda Time - its DateTimeZone class has nextTransition and previousTransition methods.

Note that transitions can occur for reasons other than DST - a change in standard time, or perhaps a change in the name of the zone interval. But that's how you'd find out, anyway :)

(Joda Time is a much better date and time library to start with, to be honest.)

Answer 2

java.time

The java.util Date-Time API and their formatting API, SimpleDateFormat are outdated and error-prone. It is recommended to stop using them completely and switch to the modern Date-Time API^*.

Also, quoted below is a notice from the home page of Joda-Time:

Note that from Java SE 8 onwards, users are asked to migrate to java.time (JSR-310) - a core part of the JDK which replaces this project.

Solution using java.time, the modern Date-Time API: Use ZoneRules#nextTransition and ZoneRules#previousTransition.

Demo:

import java.time.Instant;
import java.time.ZoneId;
import java.time.zone.ZoneRules;

public class Main {
    public static void main(String[] args) {
        Instant now = Instant.now();
        ZoneRules zoneRules = ZoneId.of("Europe/London").getRules();

        System.out.println(zoneRules.nextTransition(now));

        System.out.println(zoneRules.previousTransition(now));
    }
}

Output:

Transition[Overlap at 2021-10-31T02:00+01:00 to Z]
Transition[Gap at 2021-03-28T01:00Z to +01:00]

ONLINE DEMO

You can understand the output from the following images:

From the image, you can see that on 28 Mar 2021, after 00:59, the time changed to 02:00 i.e. there was no 01:00 that night. Is there a way to know the local time and the zone offset just before the transition happened? Check ZoneOffsetTransition or the following demo for the answer.

What were the local time and the zone offset before the transition?

import java.time.Instant;
import java.time.ZoneId;
import java.time.zone.ZoneOffsetTransition;

public class Main {
    public static void main(String[] args) {
        ZoneId zoneId = ZoneId.of("Europe/London");
        ZoneOffsetTransition nextTransition = zoneId.getRules().previousTransition(Instant.now());
        System.out.println("Local date-time before the transition: " + nextTransition.getDateTimeBefore());
        System.out.println("Zone offset before the transition: " + nextTransition.getOffsetBefore());
    }
}

Output:

Local date-time before the transition: 2021-03-28T01:00
Zone offset before the transition: Z

ONLINE DEMO

Learn more about the modern Date-Time API from Trail: Date Time.

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^{* For any reason, if you have to stick to Java 6 or Java 7, you can use ThreeTen-Backport which backports most of the java.time functionality to Java 6 & 7. If you are working for an Android project and your Android API level is still not compliant with Java-8, check Java 8+ APIs available through desugaring and How to use ThreeTenABP in Android Project.}