I need to convert from one timezone to another timezone in my project.

I am able to convert from my current timezone to another but not from a different timezone to another.

For example I am in India, and I am able to convert from India to US using Date d=new Date(); and assigning it to a calendar object and set开发者_开发知识库ting the time zone.

However, I cannot do this from different timezone to another timezone. For example, I am in India, but I am having trouble converting timezones from the US to the UK.

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tl;dr

ZonedDateTime.now( ZoneId.of( "Pacific/Auckland" ))              // Current moment in a particular time zone.
             .withZoneSameInstant( ZoneId.of( "Asia/Kolkata" ))  // Same moment adjusted into another time zone. 

Details

The java.util.Date class has no time zone assigned^†, yet it's toString implementation confusingly applies the JVM's current default time zone.

Avoid java.util.Date & .Calendar

This is one of many reasons to avoid the notoriously troublesome java.util.Date, .Calendar, and SimpleDateFormat classes bundled with Java. Avoid them. Instead use either:

• The java.time package built into Java 8 and inspired by Joda-Time.
• Joda-Time

java.time

Java 8 and later has the java.time package built-in. This package was inspired by Joda-Time. While they share some similarities and class names, they are different; each has features the other lacks. One notable difference is that java.time avoids constructors, instead uses static instantiation methods. Both frameworks are led by the same man, Stephen Colbourne.

Much of the java.time functionality has been back-ported to Java 6 & 7 in the ThreeTen-Backport project. Further adapted to Android in the ThreeTenABP project.

In the case of this Question, they work in the same fashion. Specify a time zone, and call a now method to get current moment, then create a new instance based on the old immutable instance to adjust for time zone.

Note the two different time zone classes. One is a named time zone including all the rules for Daylight Saving Time and other such anomalies plus an offset from UTC while the other is only the offset.

ZoneId zoneMontréal = ZoneId.of("America/Montreal"); 
ZonedDateTime nowMontréal = ZonedDateTime.now ( zoneMontréal );

ZoneId zoneTokyo = ZoneId.of("Asia/Tokyo"); 
ZonedDateTime nowTokyo = nowMontréal.withZoneSameInstant( zoneTokyo );

ZonedDateTime nowUtc = nowMontréal.withZoneSameInstant( ZoneOffset.UTC );

Joda-Time

Some example code in Joda-Time 2.3 follows. Search StackOveflow for many more examples and much discussion.

DateTimeZone timeZoneLondon = DateTimeZone.forID( "Europe/London" );
DateTimeZone timeZoneKolkata = DateTimeZone.forID( "Asia/Kolkata" );
DateTimeZone timeZoneNewYork = DateTimeZone.forID( "America/New_York" );

DateTime nowLondon = DateTime.now( timeZoneLondon ); // Assign a time zone rather than rely on implicit default time zone.
DateTime nowKolkata = nowLondon.withZone( timeZoneKolkata );
DateTime nowNewYork = nowLondon.withZone( timeZoneNewYork );
DateTime nowUtc = nowLondon.withZone( DateTimeZone.UTC );  // Built-in constant for UTC.

We have four representations of the same moment in the timeline of the Universe.

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^†Actually the java.util.Date class does have a time zone buried within its source code. But the class ignores that time zone for most practical purposes. So, as shorthand, it’s often said that j.u.Date has no time zone assigned. Confusing? Yes. Avoid the mess that is j.u.Date and go with Joda-Time and/or java.time.

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Some examples

Convert time between timezone

Converting Times Between Time Zones

import java.util.Calendar;
import java.util.GregorianCalendar;
import java.util.TimeZone;

public class TimeZoneExample {
    public static void main(String[] args) {
        // Create a calendar object and set it time based on the local
        // time zone
        Calendar localTime = Calendar.getInstance();
        localTime.set(Calendar.HOUR, 17);
        localTime.set(Calendar.MINUTE, 15);
        localTime.set(Calendar.SECOND, 20);

        int hour = localTime.get(Calendar.HOUR);
        int minute = localTime.get(Calendar.MINUTE);
        int second = localTime.get(Calendar.SECOND);


        // Print the local time
        System.out.printf("Local time  : %02d:%02d:%02d\n", hour, minute, second);


        // Create a calendar object for representing a Germany time zone. Then we
        // wet the time of the calendar with the value of the local time

        Calendar germanyTime = new GregorianCalendar(TimeZone.getTimeZone("Europe/Berlin"));
        germanyTime.setTimeInMillis(localTime.getTimeInMillis());
        hour = germanyTime.get(Calendar.HOUR);
        minute = germanyTime.get(Calendar.MINUTE);
        second = germanyTime.get(Calendar.SECOND);


        // Print the local time in Germany time zone
        System.out.printf("Germany time: %02d:%02d:%02d\n", hour, minute, second);
    }
}

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    Date date = new Date();
    String formatPattern = ....;
    SimpleDateFormat sdf = new SimpleDateFormat(formatPattern);

    TimeZone T1;
    TimeZone T2;

    // set the Calendar of sdf to timezone T1
    sdf.setTimeZone(T1);
    System.out.println(sdf.format(date));

    // set the Calendar of sdf to timezone T2
    sdf.setTimeZone(T2);
    System.out.println(sdf.format(date));

    // Use the 'calOfT2' instance-methods to get specific info
    // about the time-of-day for date 'date' in timezone T2.
    Calendar calOfT2 = sdf.getCalendar();

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The "default" time zone can be avoided entirely by just setting the time zone appropriately for the Calendar object. However, I would personally suggest that you use Joda Time as a far superior API for date and time operations in Java. Amongst other things, time zone conversion is very simple in Joda.

It's not clear what your current code looks like and why you're only able to convert via the default time zone, but in Joda Time you'd just specify the time zone explicitly when creating (say) a DateTime object, and then use withZone(DateTimeZone zone).

If you could tell us more about how you're getting input data, we could give a fuller example.

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You can use the following code snippet

String dateString = "14 Jul 2014 00:11:04 CEST";
date = formatter.parse(dateString);
System.out.println(formatter.format(date));

// Set the formatter to use a different timezone - Indochina Time
formatter.setTimeZone(TimeZone.getTimeZone("Asia/Bangkok"));
System.out.println("ICT time : "+formatter.format(date));

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If you don't want to use Joda, here is a deterministic way using the built in libraries.

First off I recommend that you force your JVM to default to a timezone. This addresses the issues you might run into as you move your JVM from one machine to another that are set to different timezones but your source data is always a particular timezone. For example, lets say your data is always PDT/PST time zone, but you run on a box that is set to UTC timezone.

The following code snippet sets the default timezone in my JVM:

 //You can either pass the JVM a parameter that 
 //enforces a TZ: java -Duser.timezone=UTC or you can do it
 //programatically like this
 TimeZone tz = TimeZone.getTimeZone("America/Los_Angeles");
 TimeZone.setDefault(tz);

Now lets say your source date is coming in as PDT/PST but you need to convert it to UTC. These are the steps:

    DateFormat dateFormatUtc = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    DateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
    dateFormatUtc.setTimeZone(TimeZone.getTimeZone("UTC"));

    String dateStrInPDT = "2016-05-19 10:00:00";
    Date dateInPDT = dateFormat.parse(dateStrInPDT);


    String dateInUtc = dateFormatUtc.format(dateInPDT);

    System.out.println("Date In UTC is " + dateInUtc);

The output would be:

Date In UTC is 2016-05-19 17:00:00

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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.

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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.}

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You could use the java.time.ZoneDateTime#ofInstant() method:

import java.time.*;

public class TimeZonesConversion {
    static ZonedDateTime convert(ZonedDateTime time, ZoneId newTimeZone) {
        return ZonedDateTime.ofInstant(
                time.toInstant(),
                newTimeZone);
    };

    public static void main(String... args) {
        ZonedDateTime mstTime = ZonedDateTime.of(LocalDateTime.now(), ZoneId.of("-07")); 
        ZonedDateTime localTime = convert(mstTime, Clock.systemDefaultZone().getZone());
        System.out.println("MST(" + mstTime + ") = " + localTime);
    }
}

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Depends on what you really mean by "converting".

It MAY be as simple as setting the time zone in the FORMATTER, and not mucking with Calendar at all.

Calendar cal = Calendar.getInstance();

TimeZone tzUTC = TimeZone.getTimeZone( "UTC" );
TimeZone tzPST = TimeZone.getTimeZone( "PST8PDT" );

DateFormat dtfmt = new SimpleDateFormat( "EEE, yyyy-MM-dd KK:mm a z" );

dtfmt.setTimeZone( tzUTC );
System.out.println( "UTC: " + dtfmt.format( cal.getTime() ));

dtfmt.setTimeZone( tzPST );
System.out.println( "PST: " + dtfmt.format( cal.getTime() ));

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This is not the answer, but could help someone trying to generate dates with same timezone and apply another timezone's offset. It is useful when your application server is running in one timezone and your database in another.

public static Date toGreekTimezone (Date date) {
  ZoneId greek = ZoneId.of(EUROPE_ATHENS);
  ZonedDateTime greekDate = ZonedDateTime.ofInstant(date.toInstant(), greek);

  ZoneId def = ZoneId.systemDefault();
  ZonedDateTime defDate = greekDate.withZoneSameLocal(def);

  return Date.from(defDate.toInstant());
}

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You can do something like this to get the current time in another time zone.

Calendar japanCal = new GregorianCalendar(TimeZone.getTimeZone("Japan"));
japanCal.setTimeInMillis(local.getTimeInMillis());

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here a story: my user in US enters a date in a web page. My server gets this as a java.util.Date object. Date objects have no notion of time zone.

so let's say user entered 11PM(== 4AM london time). For her this was 11PM US time. Your server gets this and interprets this as 11PM of JVM's timezone. but what you need is a Date object that represents 4AM.

SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
String timeStringInUS =  sdf.format("2020-05-04 23:00:00");

SimpleDateFormat dateFormatInUS = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
SimpleDateFormat dateFormatInUK = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

dateFormatInUS.setTimeZone(TimeZone.getTimeZone("America/New_York"));
dateFormatInUK.setTimeZone(TimeZone.getTimeZone("Europe/London"));

Date dateInUS = dateFormatInUS.parse(timeStringInUS);
Date dateInUK = sdf.parse(dateFormatInUK.format(dateInUS));

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public static String convertTimeBasedOnTimeZoneAndTimePattern(String dateTime,
  String fromTimeZone, String toTimeZone, String originalTimePattern, String timePattern) {

  DateTimeFormatter formatterNew = DateTimeFormatter.ofPattern(timePattern);

  DateTimeFormatter formatter = DateTimeFormatter.ofPattern(originalTimePattern);

  TemporalAccessor temporalAccessor = formatter.parse(dateTime);
  ZoneId z = ZoneId.of(fromTimeZone);
  LocalDateTime localDateTime = LocalDateTime.from(temporalAccessor);
  ZonedDateTime zonedDateTime = ZonedDateTime.of(localDateTime, z);
  Instant instant = Instant.from(zonedDateTime);

  ZonedDateTime fromZonedDateTime = instant.atZone(ZoneId.of(toTimeZone));

  String fromZoneDateTime = fromZonedDateTime.format(formatterNew);

return fromZoneDateTime;}

To convert any time to the specific timezone (for example: UTC -> local timezone and vise versa) with any time pattern you can use java.time library.

This method will take time patterns (original and required format) and timezone (original time zone and required timezone) will give String as output. you can convert String to date by using SimpleDateFormatter or also use parse method of the ZoneDateTime/Instant class.

To convert String to date:

public static final DATE_FORMAT="yyyy-MM-dd HH:mm:ss.SSSSS";
public static Date convertStringToDate(String date) {
    SimpleDateFormat formatter = new SimpleDateFormat(DATE_FORMAT);

    Date parsedDate = null;

    try {
      parsedDate = formatter.parse(date);
    } catch (Exception e) {
      throw new DateTimeParseException("Please provide date time in proper format", null, 0, null);
    }
    return parsedDate;
  }

To convert date to String:

public String convertTextDateToDate(Date textDate) {
    // SimpleDateFormat sdf = new SimpleDateFormat("EE MMM dd HH:mm:ss z yyyy", //Locale.ENGLISH);

    SimpleDateFormat date = new SimpleDateFormat(DATE_FORMAT);
    String dateFormatted = date.format(textDate);
    return dateFormatted;
  }