One hour difference in DST

Question

I am trying to find out the day light saving start time in US which is 14th March 2021 at 2:00 AM but my code is returning the 14th March 2021 at 3:00 AM. I am not able to understand this one hour time difference. My code is listed below:

import java.sql.Timestamp;
import java.text.SimpleDateFormat;
import java.time.Instant;
import java.time.ZoneId;
import java.time.ZonedDateTime;
import java.time.temporal.ChronoUnit;
import java.time.zone.ZoneOffsetTransition;
import java.time.zone.ZoneRules; 
class DST{
public static void main(String[] args) {       
     ZoneId zoneId = ZoneId.of("America/New_York");
     ZoneRules rules = zoneId.getRules();
     ZoneOffsetTransition nextTransition = rules.nextTransition(Instant.now());
     Timestamp ts = Timestamp.valueOf(nextTransition.getInstant().atZone(zoneId).toLocalDateTime());
     SimpleDateFormat sdf = new SimpleDateFormat("MMMM d, yyyy 'at' h:mm a");
     String date = sdf.format(ts);
     system.out.println("DST time:"+ date );
    }
}

The date value is come as March 14, 2021 at 3:00 AM but it should be March 14, 2021 at 2:00 AM. I am not able to understand the difference of one hour time difference?

Answer 1

I think the (unnecessary) conversion to Timestamp is the cause of the error:

import java.time.Instant;
import java.time.ZoneId;
import java.time.format.DateTimeFormatter;
import java.time.zone.ZoneOffsetTransition;
import java.time.zone.ZoneRules;

public class DST {
    public static void main(String[] args) {
        ZoneId zoneId = ZoneId.of("America/New_York");
        ZoneRules rules = zoneId.getRules();
        ZoneOffsetTransition nextTransition = rules.nextTransition(Instant.now());
        DateTimeFormatter sdf = DateTimeFormatter.ofPattern("MMMM d, yyyy 'at' h:mm a");
        System.out.println("DST time:" + nextTransition.getDateTimeBefore().format(sdf));
    }
}

gives you (as expected): March 14, 2021 at 2:00 AM

At this moment in time the time change already occurred, so it's indeed 3AM. That's the reason why API designers had to create the method getDateTimeBefore().

Answer 2

Check the following information from https://www.timeanddate.com/time/change/usa/new-york

14 Mar 2021 - Daylight Saving Time Starts
When local standard time is about to reach
Sunday, 14 March 2021, 02:00:00 clocks are turned forward 1 hour to
Sunday, 14 March 2021, 03:00:00 local daylight time instead.

The information matches with the output of the following code:

import java.sql.Timestamp;
import java.text.SimpleDateFormat;
import java.time.LocalDateTime;
import java.time.ZoneId;
import java.time.ZoneOffset;
import java.time.ZonedDateTime;
import java.time.temporal.ChronoUnit;
import java.time.zone.ZoneOffsetTransition;
import java.time.zone.ZoneRules;
import java.util.Date;

public class Main {
    public static void main(String[] args) {
        ZoneId zoneId = ZoneId.of("America/New_York");

        ZoneRules rules = zoneId.getRules();
        ZonedDateTime zdt = ZonedDateTime.now(zoneId);
        ZoneOffsetTransition nextTransition = rules.nextTransition(zdt.toInstant());

        ZonedDateTime zdtAfterTransition = nextTransition.getInstant().atZone(zoneId);
        ZonedDateTime zdtBeforeTransition = nextTransition.getInstant().minus(1, ChronoUnit.DAYS).atZone(zoneId);
        System.out.println(zdtBeforeTransition);
        System.out.println(zdtAfterTransition);

        LocalDateTime ldtBeforeTransition = nextTransition.getDateTimeBefore();
        System.out.println(ldtBeforeTransition);

        // Convert LocalDateTime to java.sql.Timestamp
        Timestamp ts = new Timestamp(Date.from(ldtBeforeTransition.toInstant(ZoneOffset.UTC)).getTime());
        SimpleDateFormat sdf = new SimpleDateFormat("MMMM d, yyyy 'at' h:mm a");
        String date = sdf.format(ts);
        System.out.println("DST time:" + date);
    }
}

Output:

2021-03-13T02:00-05:00[America/New_York]
2021-03-14T03:00-04:00[America/New_York]
2021-03-14T02:00
DST time:March 14, 2021 at 2:00 am

The date-time API of java.util and their formatting API, SimpleDateFormat are outdated and error-prone. The class, java.sql.Timestamp extends java.util.Date and therefore it inherits the same problems. It is recommended to stop using them completely and switch to the modern date-time API.

• 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.

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