CODEWITHSUNDEEP
javadstdatetimeoffset
gelgamil
gelgamil

Reputation: 961

Determine Whether Daylight Savings Time (DST) is Active in Java for a Specified Date

I have a Java class that takes in the latitude/longitude of a location and returns the GMT offset when daylight savings time is on and off. I am looking for an easy way to determine in Java if the current date is in daylight savings time so I can apply the correct offset. Currently I am only performing this calculation for U.S. timezones although eventually I would like to expand this to global timezones as well.

Upvotes: 78

Views: 123938

Answers (6)

user16139739
user16139739

Reputation: 1155

  1. I'm not sure what you'd be doing with the offset, but the getOffset() method of the TimeZone class will get the offset, without having to find out whether the time being processed is in daylight saving time or not - it does all that work for you. You may even be able to just use the ZonedDateTime class.
  2. There is a project that generates a shapefile of IANA timezone boundaries, which might let you map from latitude/longitude to a timezone ID.

Upvotes: 0

Basil Bourque
Basil Bourque

Reputation: 338730

tl;dr

ZoneId.of( "America/Montreal" )  // Represent a specific time zone, the history of past, present, and future changes to the offset-from-UTC used by the people of a certain region.  
      .getRules()                // Obtain the list of those changes in offset. 
      .isDaylightSavings(        // See if the people of this region are observing Daylight Saving Time at a specific moment.
          Instant.now()          // Specify the moment. Here we capture the current moment at runtime. 
      )                          // Returns a boolean.

java.time

Here is the modern java.time (see Tutorial) version of the correct Answer by mamboking.

Example code:

ZonedDateTime now = ZonedDateTime.now( ZoneId.of( "America/Montreal" ) );
…
ZoneId z = now.getZone();
ZoneRules zoneRules = z.getRules();
Boolean isDst = zoneRules.isDaylightSavings( now.toInstant() );

Note how in the last line we had to extract an Instant object from our ZonedDateTime object with a simple call to toInstant.

Table of date-time types in Java, both modern and legacy.

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.

You may exchange java.time objects directly with your database. Use a JDBC driver compliant with JDBC 4.2 or later. No need for strings, no need for java.sql.* classes.

Where to obtain the java.time classes?

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.

Upvotes: 66

Clint
Clint

Reputation: 9058

This is the answer for the machine on which the question is being asked:

TimeZone.getDefault().inDaylightTime( new Date() );

A server trying to figure this out for a client will need the client's time zone. See @Powerlord answer for the reason why.

For any particular TimeZone 

TimeZone.getTimeZone( "US/Alaska").inDaylightTime( new Date() );

Upvotes: 89

Powerlord
Powerlord

Reputation: 88796

You're going to have to do a bit more work using those coordinates and figure out which time zone they're in. Once you know which TimeZone that is, the isDayLight() method would be useful.

For example, you have no way of telling whether -0500 is EST (US/Canada Eastern Standard Time), CDT (US/Canada Central Daylight Time), COT (Colombia Time), AST (Brazil Acre Standard Time), ECT (Ecuador Time), etc...

Some of these may or may not support daylight saving time.

Upvotes: 3

James Van Huis
James Van Huis

Reputation: 5571

Joda Time contains handling methods which will calculate the offsets for you. See DateTimeZone.convertLocalToUTC(...)

To supplement this, you will need to look up the current time zone with your latitude/longitude info. GeoNames provides a java client for its web service, as well as a simple web-request framework (i.e. http://ws.geonames.org/timezone?lat=47.01&lng=10.2)

Upvotes: 1

mamboking
mamboking

Reputation: 4637

TimeZone tz = TimeZone.getTimeZone("EST");
boolean inDs = tz.inDaylightTime(new Date());

Upvotes: 10

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