Hi I have a following code below:

public Date convertFromGMTToLocal(Date date) {
     return new Date(date.getTimezoneOffset() + newOffset*60*1000);  
}

public Date convertFromLocalToGMT(Date date) {
    return new Date(date.getTime() - date.getTimezoneOffset()*60*1000);
}

convertFromLocalToGMT is supposed to strip off timezone information and convertFromGMTToLocal is supposed to put the timezone information back. I understand java.util.Date is representing the time in epoch and always in GMT 开发者_运维问答however when displaying the date it is defaulted to JVM's default timezone.

Explanation I got was that if your timezone is CST and it is 10:00AM CST you are changing timezone to GMT with convertFromLocalToGMT so you're essentially adding the offset to get GMT so you get 4:00AM CST (offset is -6) and using convertFromGMTToLocal will convert this Date object back to 10:00AM regardless of your timezones (the most confusing part how?). How does above work? I am confused...

Thanks.

You should pretty much never add an offset to a Date to create another Date - you'd only ever do that if you received broken data to start with.

You should not use this code. It is bad code which tries to treat java.util.Date in a way it was not designed for. If you want to represent a date in a particular time zone, either use Calendar (urgh) or the far better Joda Time API.

In particular, the code you've got will not work around time zone transitions - because the offset at date.getTimeZoneOffset() still considers date to be UTC (because that's what it's defined as) even you're treating it as a local date/time.

Ignore the value that's displayed by Date.toString() - avoid using that method. Either display using SimpleDateFormat with appropriate settings for the time zone you're interested in, or (better, again) use Joda's DateTimeFormat class.

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The java.util Date-Time API and their formatting API, SimpleDateFormat are outdated and error-prone.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.

You can solve it using the following functions in java.time, the modern Date-Time API :

1. LocalDateTime#atZone(ZoneId)
2. ZonedDateTime#withZoneSameInstant(ZoneId)

Demo:

import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.LocalTime;
import java.time.ZoneId;
import java.time.ZonedDateTime;

/*
 * Change the JVM's ZoneId, ZoneId.systemDefault() to the required ZoneId e.g. ZoneId.of("America/New_York")
 */
public class Main {
    public static void main(String[] args) {
        // Test
        ZoneId zoneId = ZoneId.of("Etc/UTC");
        ZonedDateTime zdt = ZonedDateTime.of(LocalDate.of(2021, 5, 10), LocalTime.of(15, 20), zoneId);
        System.out.println(zdt + " is " + convertFromZdtToLdt(zdt) + " at " + ZoneId.systemDefault().getId());

        LocalDateTime ldt = LocalDateTime.of(LocalDate.of(2021, 5, 10), LocalTime.of(15, 20));
        System.out
                .println(ldt + " at " + ZoneId.systemDefault().getId() + " is " + convertFromLdtZdtToZdt(ldt, zoneId));
    }

    public static LocalDateTime convertFromZdtToLdt(ZonedDateTime zdt) {
        return zdt.withZoneSameInstant(ZoneId.systemDefault()).toLocalDateTime();
    }

    public static ZonedDateTime convertFromLdtZdtToZdt(LocalDateTime ldt, ZoneId targetZoneId) {
        return ldt.atZone(ZoneId.systemDefault()).withZoneSameInstant(targetZoneId);
    }
}

Output on my system in Europe/London timezone:

2021-05-10T15:20Z[Etc/UTC] is 2021-05-10T16:20 at Europe/London
2021-05-10T15:20 at Europe/London is 2021-05-10T14:20Z[Etc/UTC]

The Z in the output is the timezone designator for zero-timezone offset. It stands for Zulu and specifies the Etc/UTC timezone (which has the timezone offset of +00:00 hours).

ONLINE DEMO

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

^{* 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.}

As you said, the Date object doesn't care about timezone. It stores an instant in time. The timezone is only meaningful when you want to display a date. Use a DateFormat with the specific timezone you want to use to display a date (using setTimeZone)