How to trim minutes and hours and seconds from Date object?

Question

I need to make map where Dates are keys. 2 date objects are equals if they have the same value of getTime() method.

I'm interested only in year, month and day. How can I trim unnecessary hours and minutes to get 'clear' dates?

Answer 1

tl;dr

LocalDate ld =
    myUtilDate.toInstant()
              .atZone( ZoneId.of( "America/Montreal" ) )
              .toLocalDate();

Details

The Question and other Answers use outmoded old date-time classes that have proven to be poorly-designed, confusing, and troublesome. Now legacy, supplanted by the java.time classes.

Instant truncated

To more directly address the question:

1. Convert to java.time, from java.util.Date to java.time.Instant.
2. Truncate to the date.

Convert via new methods added to the old classes.

Instant instant = myUtilDate.toInstant();

The truncation feature is built into the Instant class. 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 instantTruncated = instant.truncatedTo( ChronoUnit.DAYS );

ZonedDateTime & LocalDate

But the approach above has issues. Both java.util.Date and Instant represent a moment on the timeline in UTC rather than an particular time zone. So if you drop the time-of-day, or set it to 00:00:00, you are getting a date that only makes sense in UTC. If you meant the date for Auckland NZ or Montréal Québec, you may have the wrong date.

So a better approach is to apply your desired/expected time zone to the Instant to get a ZonedDateTime.

Another problem is that we are inappropriately using a date-time object to represent a date-only value. Instead we should use a date-only class. From the ZonedDateTime we should extract a LocalDate if all you want is the date-only.

The LocalDate class represents a date-only value without time-of-day and without time zone.

ZoneId z = ZoneId.of( "America/Montreal" );
ZonedDateTime zdt = instant.atZone( z );
LocalDate ld = zdt.toLocalDate();

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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 java.time.

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….

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.