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The java.util Date-Time API and their formatting API, SimpleDateFormat are outdated and error-prone.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 at 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.
How to switch to the modern API?
Convert java.util.Date to Instant using Date#toInstant e.g.
Date date = new Date();
Instant instant = date.toInstant();
System.out.println(instant); // 2021-05-30T13:10:01.890Z
What's Instant got to do with my requirement?
An Instant represents an instantaneous point on the timeline in UTC. The Z in the sample output is the timezone designator for a zero-timezone offset. It stands for Zulu and specifies the Etc/UTC timezone (which has the timezone offset of +00:00 hours). Its zero-timezone offset makes it independent of timezones i.e. an instant is the same at every place in the world. It's analogous to water in the physical world.
You can mix a timezone (i.e. ZoneId) with an Instant by calling Instant.atZone to get the corresponding Date-Time in that timezone (i.e. ZonedDateTime).
Similarly, you can mix a timezone offset (i.e. ZoneOffset) with an Instant by calling Instant#atOffset to get the corresponding Date-Time with that timezone offset (i.e. OffsetDateTime).
In the reverse way, you can also get an Instant by calling toInstant on the ZonedDateTime or OffsetDateTime.
import java.time.Instant;
import java.time.OffsetDateTime;
import java.time.ZoneId;
import java.time.ZoneOffset;
import java.time.ZonedDateTime;
import java.util.Date;
public class Main {
public static void main(String[] args) {
Date date = new Date();
Instant instant = date.toInstant();
System.out.println(instant);
// The corresponding Date-Time in Chicago
ZonedDateTime zdtChicago = instant.atZone(ZoneId.of("America/Chicago"));
System.out.println(zdtChicago);
// The corresponding Date-Time in Kolkata
ZonedDateTime zdtKolkata = instant.atZone(ZoneId.of("Asia/Kolkata"));
System.out.println(zdtKolkata);
// The corresponding Date-Time at timezone offset of -05:00 hours
OffsetDateTime odtAtOffsetMinus0500 = instant.atOffset(ZoneOffset.of("-05:00"));
System.out.println(odtAtOffsetMinus0500);
// The corresponding Date-Time at timezone offset of +05:30 hours
OffsetDateTime odtAtOffset0530 = instant.atOffset(ZoneOffset.of("+05:30"));
System.out.println(odtAtOffset0530);
}
}
Output:
2021-05-30T13:44:26.599Z
2021-05-30T08:44:26.599-05:00[America/Chicago]
2021-05-30T19:14:26.599+05:30[Asia/Kolkata]
2021-05-30T08:44:26.599-05:00
2021-05-30T19:14:26.599+05:30
So far you have learnt a simple way to convert an Instant (which you have created directly or obtained from a java.util.Date or a ZonedDateTime or an OffsetDateTime) to a Date-Time in any timezone or at any timezone offset.
Alternatively
There is another way to convert a ZonedDateTime from one timezone to another. Again, there is a similar method to convert an OffsetDateTime from one timezone offset to another.
import java.time.OffsetDateTime;
import java.time.ZoneId;
import java.time.ZoneOffset;
import java.time.ZonedDateTime;
public class Main {
public static void main(String[] args) {
// Current Date-Time in Chicago
ZonedDateTime zdtChicago = ZonedDateTime.now(ZoneId.of("America/Chicago"));
System.out.println(zdtChicago);
// The corresponding Date-Time in Kolkata
ZonedDateTime zdtKolkata = zdtChicago.withZoneSameInstant(ZoneId.of("Asia/Kolkata"));
System.out.println(zdtKolkata);
// Current Date-Time at a timezone offset of -05:00 hours
OffsetDateTime odtAtOffsetMinus0500 = OffsetDateTime.now(ZoneOffset.of("-05:00"));
System.out.println(odtAtOffsetMinus0500);
// The corresponding Date-Time at timezone offset of +05:30 hours
OffsetDateTime odtAtOffset0530 = odtAtOffsetMinus0500.withOffsetSameInstant(ZoneOffset.of("+05:30"));
System.out.println(odtAtOffset0530);
}
}
Output:
2021-05-30T10:03:59.895923-05:00[America/Chicago]
2021-05-30T20:33:59.895923+05:30[Asia/Kolkata]
2021-05-30T10:03:59.897782-05:00
2021-05-30T20:33:59.897782+05:30
When to use ZonedDateTime and when to use OffsetDateTime?
- If you are dealing with a fixed timezone offset value e.g.
02:00 hours, use OffsetDateTime. It is also supported by all JDBC drivers. Check this answer to learn more about it.
- If you want the timezone offset to change automatically based on DST, use
ZonedDateTime. Unfortunately, ZonedDateTime is not supported by JDBC.
Learn more about java.time, 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.
