Java Calendar timezone converting to GMT problem

Question

I have one Calendar object which is as per the user's time zone which may be PST etc, now i want to convert the same to GMT and retain the time ie is the calendar initially was set @ 00:00:00 at PST it should be converted to 08:00:00 after the conversion taking into consideration the time/date difference. Can someone provide me some help on this.

Appreciate the help in advance.

Thanks,

Vaibhav

Answer 1

Just create a new Calendar in GMT, set the time in that calendar to the same as the original calendar, and you're done:

gmtCalendar.setTime(userCalendar.getTime());

That should be fine, as the getTime() call returns the instant in time (ie a java.util.Date with no associated time zone).

As ever though, if you're doing any significant amount of date/time work in Java you should strongly consider using Joda Time instead.

Answer 2

tl;dr

( ( GregorianCalendar ) myCal )  // Cast from a general `Calendar`  to  specific subclass `GregorianCalendar`. 
    .toZonedDateTime()           // Convert from troublesome legacy class to modern java.time class, `ZonedDateTime`.
    .toInstant()                 // Extract a UTC-specific value, an `Instant` object.

java.time

The modern approach uses java.time classes.

Convert your legacy Calendar object (if GregorianCalendar ) to a ZonedDateTime . Call new conversion methods added to the old classes.

GregorianCalendar gc = ( GregorianCalendar ) myCal ;
ZonedDateTime zdt = gc.toZonedDateTime() ;

Now extract an Instant , a value always in UTC. You can think of it this way conteptually: ZonedDateTime = ( Instant + ZoneId )

Instant instant = zdt.toInstant() ;

For more flexibility such as generating strings in various formats, convert to an OffsetDateTime object.

OffsetDateTime odt = instant.atOffset( ZoneOffset.UTC ) ;

---

About java.time

The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date , Calendar , & SimpleDateFormat .

The Joda-Time project, now in maintenance mode , advises migration to the java.time classes.

To learn more, see the Oracle Tutorial . And search Stack Overflow for many examples and explanations. Specification is JSR 310 .

Using a JDBC driver compliant with JDBC 4.2 or later, you may exchange java.time objects directly with your database. No need for strings nor java.sql.* classes.

Where to obtain the java.time classes?

• Java SE 8 , Java SE 9 , and later
  • Built-in.
  • Part of the standard Java API with a bundled implementation.
  • Java 9 adds some minor features and fixes.
• Java SE 6 and Java SE 7
  • Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport .
• Android
  • Later versions of Android bundle implementations of the java.time classes.
  • For earlier Android, the ThreeTenABP project adapts ThreeTen-Backport (mentioned above). See How to use ThreeTenABP… .

The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval , YearWeek , YearQuarter , and more .