What is the correct way to create a timestamp in Java?

Question

Given a year, month, day, hour, minute and second, what is the correct way to create a Java UTC (GMT) timestamp?

Options I've considered, but remain to be convinced by:

1 - use deprecated Date constructors

java.util.Date date = new java.util.Date(year - 1900, month, dayOfMonth, hour, minute, second);
long timestamp = date.getTime();

2 - use a calendar with TimeZone set to GMT

Calendar c = Calendar.getInstance(TimeZone.getTimeZone("GMT")); 
c.set(Calendar.YEAR, year);
c.set(Calendar.MONTH, month);
c.set(Calendar.DAY_OF_MONTH, dayOfMonth);
c.set(Calendar.HOUR_OF_DAY, hour);
c.set(Calendar.MINUTE, minute);
c.set(Calendar.SECOND, second);
c.set(Calendar.MILLISECOND, 0);
long timestamp = c.getTimeInMillis();

One of my issues with this is that I'm finding it very hard to test without getting mixed up in more TimeZone issues.

Is there a definitive right way to do this with the standard APIs?

Update: would like to get an answer to this using standard JavaSE. I know Joda-Time is wonderful, but it's not an option in this case.

Answer 1

tl;dr

OffsetDateTime.of( 2018 , 1 , 23 , 12 , 34 , 56 , 0, ZoneOffset.UTC )

java.time

The modern approach uses the java.time classes.

Unlike the troublesome legacy date-time classes, the java.time classes use sane numbering:

• 2018 means the year 2018. (No crazy math with 1900.)
• 1-12 for months January-December. (Not silly 0-11.)
• 1-7 for Monday-Sunday per ISO 8601 standard definition of week. (No varying by locale.)

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).

You a one-liner if you wish, if parsing a string.

Instant instant = Instant.parse( "2018-01-23T12:34:56Z" ) ;

While Instant is a basic building-block class for java.time, the OffsetDateTime class is more flexible. The offset-from-UTC of UTC itself is defined as a constant for your convenience.

OffsetDateTime odt = OffsetDateTime.of( 2018 , 1 , 23 , 12 , 34 , 56 , 0, ZoneOffset.UTC ) ;

Personally, I prefer using pieces.

In place of a mere integer for month, you may specify a Month enum object.

LocalDate ld = LocalDate.of( 2018 , Month.JANUARY , 23 ) ;  // Date-only, without time-of-day.
LocalTime lt = LocalTime.of( 12 , 34 , 56 ) ;  // Time-of-day, without date.
ZoneOffset offset = ZoneOffset.UTC ;
OffsetDateTime odt = OffsetDateTime.of( ld , lt , offset ) ;

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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.

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.

Answer 2

No, there isn't (unless you can use Joda-Time), the correct way is to use Calendar and the calendar API is ugly, nothing anyone can do about it at the moment.

Answer 3

Standard Java APIs for dates are pretty inconvenient. Consider using Joda-Time:

long timestamp = new LocalDateTime(year, month, day, hour, minute, second)
    .getLocalMillis();

Answer 4

I think this would work pretty good

SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ssZ");
Date date = sdf.parse("2011-12-21 12:00:00+0000");

You would have to build the date string on your own but it's short and should work fine. And create the SimpleDateFormat to your liking, the important part is the Z that corresponds to "+0000".

Here is another alternative way on the same lines

SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
sdf.setTimeZone(TimeZone.getTimeZone("UTC"));
Date date = sdf.parse("2011-12-21 12:00:00");

It's with an explicit time zone and the Z is removed.