Convert java.util.Date to what “java.time” type?

Question

I have a java.util.Date object, or a java.util.Calendar object. How do I convert that to the right type in java.time framework?

I have heard that we should now be doing the bulk of our business logic with java.time types. When working with old code not yet updated for java.time I need to be able to convert back and forth. What types map to java.util.Date or java.util.Calendar?

Answer 1

Yes, you definitely should be using the java.time framework whenever possible.

Avoid old date-time classes

The old date-time classes including java.util.Date, java.util.Calendar, and java.text.SimpleTextFormat and such have proven to be poorly designed, confusing, and troublesome. Avoid them where you can. But when you must interoperate with these old types, you can convert between old and new.

Read on for a basic introduction, somewhat over-simplified, to orient you in moving back-and-forth between the old and new date-time classes.

java.time

The java.time framework is defined by JSR 310, inspired by the highly-successful Joda-Time library, and extended by the ThreeTen-Extra project. The bulk of the functionality was back-ported to Java 6 & 7 in the ThreeTen-Backport project, with a further adaptation for Android in the ThreeTenABP project.

What java.time type matches java.util.Date? Well, a java.util.Date object basically represents a moment on the timeline in UTC, a combination of a date and a time-of-day. We can translate that to any of several types in java.time. Each is discussed below. Note that some new methods have been added to the old date-time classes to facilitate conversions.

Instant

The building block in java.time is an Instant, a moment on the timeline in UTC with a resolution of nanoseconds.

Generally you should do much of your business logic in UTC. In such work, Instant will be used frequently. Pass around Instant objects, applying a time zone only for presentation to a user. When you do need to apply an offset or time zone, use the types covered further below.

From java.util.Date to Instant

Given that both Instant and java.util.Date are a moment on the timeline in UTC, we can easily move from a java.util.Date to an Instant. The old class has gained a new method, java.util.Date::toInstant.

Instant instant = myUtilDate.toInstant();

You can go the other direction, from an Instant to a java.util.Date. But you may lose information about the fractional second. An Instant tracks nanoseconds, for up to nine digits after the decimal place such as 2016-01-23T12:34:56.123456789Z. Both java.util.Date & .Calendar are limited to milliseconds, for up to three digits after the decimal place such as 2016-01-23T12:34:56.123Z. In this example going from Instant to Date means truncation of the 456789.

java.util.Date myUtilDate = java.util.Date.from( instant );

From java.util.Calendar to Instant

What about a java.util.Calendar instead of a java.util.Date? Internal to the Calendar object the date-time is tracked as a count of milliseconds from the epoch reference date-time of the first moment of 1970 in UTC (1970-01-01T00:00:00.0Z). So this value can be converted easily to an Instant.

Instant instant = myUtilCalendar.toInstant() ;

From java.util.GregorianCalendar to ZonedDateTime

Even better, if your java.util.Calendar object is actually a java.util.GregorianCalendar you can easily go directly to a ZonedDateTime. This approach has the benefit of retaining the embedded time zone information.

Downcast from the interface of Calendar to the concrete class of GregorianCalendar. Then call the toZonedDateTime and from methods to go back and forth.

if( myUtilCalendar instanceof GregorianCalendar ) {
    GregorianCalendar gregCal = (GregorianCalendar) myUtilCalendar; // Downcasting from the interface to the concrete class.
    ZonedDateTime zdt = gregCal.toZonedDateTime();  // Create `ZonedDateTime` with same time zone info found in the `GregorianCalendar`
end if 

Going the other direction…

    java.util.Calendar myUtilCalendar = java.util.GregorianCalendar.from( zdt ); // Produces an instant of `GregorianCalendar` which implements `Calendar` interface.

As discussed above, beware that you may be losing information about the fraction of a second. The nanoseconds in the java.time type (ZonedDateTime) gets truncated to milliseconds in the .Calendar/.GregorianCalendar.

OffsetDateTime

From an Instant we can apply an offset-from-UTC to move into a wall-clock time for some locality. An offset is a number of hours, and possibly minutes and seconds, ahead of UTC (eastward) or behind UTC (westward). The ZoneOffset class represents this idea. The result is an OffsetDateTime object.

ZoneOffset offset = ZoneOffset.of( "-04:00" ); 
OffsetDateTime odt = OffsetDateTime.ofInstant( instant , zoneOffset );

You can go the other direction, from an OffsetDateTime to a java.util.Date. Extract an Instant and then proceed as we saw in code above. As discussed above, any nanoseconds get truncated to milliseconds (data loss).

java.util.Date myUtilDate = java.util.Date.from( odt.toInstant() );

ZonedDateTime

Better yet, apply a full time zone. A time zone is an offset plus rules for handling anomalies such as Daylight Saving Time (DST).

Applying a ZoneId gets you a ZonedDateTime object. Use a proper time zone name (continent/region). Never use the 3-4 letter abbreviations commonly seen such as EST or IST as they are neither standardized nor unique.

ZoneId zoneId = ZoneId.of( "America/Montreal" );
ZonedDateTime zdt = ZonedDateTime.ofInstant( instant , zoneId );

You can go the other direction, from an ZonedDateTime to a java.util.Date. Extract an Instant and then proceed as we saw in code above. As discussed above, any nanoseconds get truncated to milliseconds (data loss).

java.util.Date myUtilDate = java.util.Date.from( zdt.toInstant() );

And we saw further above that a ZonedDateTime may be converted to a GregorianCalendar.

LocalDate

Sometimes you may want a date-only value, without time-of-day and without time zone. For that, use a java.time.LocalDate object.

See this Question for more discussion, Convert java.util.Date to java.time.LocalDate, especially this Answer written by the main man behind the invention of both Joda-Time and java.time.

The key is to go through a ZonedDateTime (as generated in code above). We need a time zone to determine a date. The date varies around the world, with a new day dawning earlier in the east. For example, after midnight in Paris is a new day while still “yesterday” in Montréal. So while a LocalDate does not contain a time zone, a time zone is required to determine a LocalDate.

LocalDate localDate = zdt.toLocalDate();

Converting in the other direction from LocalDate to a date-time means inventing a time-of-day. You can choose any time-of-day that makes sense in your business scenario. For most people, the first moment of the day makes sense. You may be tempted to hard code that first moment as the time 00:00:00.0. In some time zones, that time may not be valid as the first moment because of Daylight Saving Time (DST) or other anomalies. So let java.time determine the correct time with a call to atStartOfDay.

ZonedDateTime zdt = localDate.atStartOfDay( zoneId );

LocalTime

On rare occasion you may want only a time-of-day without a date and without a time zone. This concept is represented by the LocalTime class. As discussed above with LocalDate, we need a time zone to determine a LocalTime even though the LocalTime object does not contain (does not ‘remember’) that time zone. So, again, we go through a ZonedDateTime object obtained from an Instant as seen above.

LocalTime localTime = zdt.toLocalTime();

LocalDateTime

As with the other two Local… types, a LocalDateTime has no time zone nor offset assigned. As such you may rarely use this. It gives you a rough idea of a date-time but is not a point on the timeline. Use this if you mean some general date and some time that might be applied to a time zone.

For example, “Christmas starts this year” would be 2016-12-25T00:00:00.0. Note the lack of any offset or time zone in that textual representation of a LocalDateTime. Christmas starts sooner in Delhi India than it does in Paris France, and later still in Montréal Québec Canada. Applying each of those areas’ time zone would yield a different moment on the timeline.

LocalDateTime ldt = zdt.toLocalDateTime();