How do I get a UTC Timestamp from Calendar?

Question

In Java when I use Calendar.getInstance(); I get a Calendar object for the current Timezone. But java.sql.Timestamp is usually stored in U

Answer 1

tl;dr

Instant.now() 

Avoid legacy date-time classes

You are using troublesome old date-time classes now supplanted by the java.time classes. No need to be using DateFormat, Calendar, or Timestamp.

If you must interopt with a Calendar from old code not yet updated to java.time, convert using new methods added to the old classes. Extract an obsolete java.util.Date, and convert to Instant.

Instant instant = myCal.getTime().toInstant() ;

Using java.time

Get the current moment as an Instant. The Instant class represents a moment on the timeline in UTC with a resolution of nanoseconds (up to nine (9) digits of a decimal fraction).

Instant instant = Instant.now() ;

2017-02-17T03:29:15.725Z

Database

With JDBC 4.2 and later, you can directly exchange a Instant object with your database. So no need for the obsolete java.sql.Timestamp.

myPreparedStatement.setObject( … , instant ) ;

And retrieval:

Instant instant = myResultSet.getObject( … , Instant.class ) ;

Count from epoch

Apparently you want a number of milliseconds since the epoch reference date of 1970-01-01T00:00:00Z. You can extract that number from an Instant. But beware of data loss, as a Instant has a resolution of nanoseconds which you will be truncating to milliseconds.

long millis = instant.toEpochMilli() ;

1487302155725

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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.

Where to obtain the java.time classes?

• Java SE 8 and 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 SE 7
  • Much of the java.time functionality is back-ported to Java 6 & 7 in ThreeTen-Backport.
• Android
  • The ThreeTenABP project adapts ThreeTen-Backport (mentioned above) for Android specifically.
  • 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.

Answer 2

When you call Calendar.getTime(), that will give you a value which doesn't have a related time zone - or you could think of it as being in UTC - for the instant that the calendar represents. So if it was (say) 9am in the calendar's time zone, it could be 5pm UTC.

Now java.util.Date (and I'd imagine Timestamp) will be formatted in the system default time zone when you call toString() (whether implicitly or explicitly) - but don't let that fool you into thinking that the time zone is part of the data within the object itself.

You need to be very clear about exactly what you're trying to achieve - and then you'll probably find that Joda Time lets you express that in code more clearly than the built-in libraries do.

Answer 3

Following snippet should satisfy your requirements:

df.setTimeZone(TimeZone.getTimeZone("UTC"));

Answer 4

Your code is already doing exactly what you want. Timestamp (as well as Date) does not have timezone information and should always contain a GMT timestamp (which ist what Calendar.getTimeInMillis() returns).

The reson why you see local time printed is that the DateFormat factory methods as well as Timestamp.toString() implicitly use the system timezone.