We can change the time zone of the local machine by changing the default timezone of the JVM like below.
TimeZone.setDefault(TimeZone.getTimeZone("NEW_TIME_ZONE");
A java.util.Date has no time zone†. It represents
UTC/GMT (no time zone offset). Very confusing because its toString method applies the JVM's default time zone when generating a String representation.
Avoid j.u.Date
For this and many other reasons, you should avoid using the built-in java.util.Date & .Calendar & java.text.SimpleDateFormat. They are notoriously troublesome.
Instead use the
java.time package bundled with
Java 8. These new classes are inspired by
Joda-Time, defined by
JSR 310, and extended by the
ThreeTen-Extra project. For Java 6 & 7, use the back-port project,
ThreeTen-Backport. For Android, the adaptation of that back-port,
ThreeTenABP. See
Oracle Tutorial.
java.time
The java.time classes can represent a moment on the timeline in three ways:
- UTC (Instant)
- With an offset (OffsetDateTime with ZoneOffset)
- With a time zone (ZonedDateTime with ZoneId)
Instant
In
java.time, the basic building block is
Instant, a moment on the time line in UTC. Use Instantobjects for much of your business logic.Instant instant = Instant.now();
OffsetDateTime
Apply an
offset-from-UTC to adjust into some locality’s
wall-clock time.
Apply a
ZoneOffset to get an
OffsetDateTime.ZoneOffset zoneOffset = ZoneOffset.of( "-04:00" );
OffsetDateTime odt = OffsetDateTime.ofInstant( instant , zoneOffset );
ZonedDateTime
Better is to apply a
time zone, an offset plus the rules for handling anomalies such as
Daylight Saving Time (DST).
Apply a
ZoneId to an Instant to get a
ZonedDateTime. Always specify a
proper time zone name. Never use 3-4 abbreviations such as EST or IST that are neither unique nor standardized.ZoneId zoneId = ZoneId.of( "America/Montreal" );
ZonedDateTime zdt = ZonedDateTime.ofInstant( instant , zoneId );
Formatted Strings
Call the toString method on any of these three classes to generate a String representing the date-time value in standard
ISO 8601 format. The ZonedDateTime class extends standard format by appending the name of the time zone in brackets.
String outputInstant = instant.toString(); // Ex: 2011-12-03T10:15:30Z
String outputOdt = odt.toString(); // Ex: 2007-12-03T10:15:30+01:00
String outputZdt = zdt.toString(); // Ex: 2007-12-03T10:15:30+01:00[Europe/Paris]
For other formats use the
DateTimeFormatter class. Generally best to let that class generate localized formats using the user’s expected human language and cultural norms. Or you can specify a particular format.
Joda-Time
While
Joda-Time is still actively maintained, its makers have told us to migrate to java.time as soon as is convenient. I leave this section intact as a reference, but I suggest using the java.time section above instead.
In
Joda-Time, a date-time object (
DateTime) truly does know its assigned time zone. That means an offset from UTC and the rules and history of that time zone’s Daylight Saving Time (DST) and other such anomalies.
String input = "2014-01-02T03:04:05";
DateTimeZone timeZone = DateTimeZone.forID( "Asia/Kolkata" );
DateTime dateTimeIndia = new DateTime( input, timeZone );
DateTime dateTimeUtcGmt = dateTimeIndia.withZone( DateTimeZone.UTC );
Call the toString method to generate a String in
ISO 8601 format.String output = dateTimeIndia.toString();
Joda-Time also offers rich capabilities for generating all kinds of other String formats.
If required, you can convert from Joda-Time DateTime to a java.util.Date.Java.util.Date date = dateTimeIndia.toDate();
Search StackOverflow for "joda date" to find many more examples, some quite detailed.
