java.time equivalent of Joda-Time `withTimeAtStartOfDay`? (get first moment of the day)

Question

In the Joda-Time library, the DateTime class offers a method withTimeAtStartOfDay to get the first moment of the day. You might think of that moment as "midnight". That first moment is usually the time 00:00:00.000 but not always.

Does the java.time package found in Java 8 and later have an equivalent feature?

Answer 1

The equivalent is using a special method, atStartOfDay, in the class LocalDate:

ZoneId zoneId = ZoneId.of("America/New_York");
ZonedDateTime zdt = LocalDate.now(zoneId).atStartOfDay(zoneId);

Please also note that the equivalent of Joda-Time DateTime is not LocalDateTime, but ZonedDateTime. The zoneId parameter matters here. Concrete example for migration - see also timezone website for details about Daylight Saving Time (DST) transition in Brazil:

Joda-Time (old way)

DateTime dt = 
  new DateTime(2015, 10, 18, 12, 0, DateTimeZone.forID("America/Sao_Paulo"));
dt = dt.withTimeAtStartOfDay();
System.out.println(dt); // 2015-10-18T01:00:00.000-02:00

Note that this code would even throw an exception for midnight in first line with call to constructor.

java.time (new way)

ZoneId zoneId = ZoneId.of("America/Sao_Paulo");
ZonedDateTime zdt =
  ZonedDateTime.of(2015, 10, 18, 12, 0, 0, 0, zoneId);
zdt = zdt.toLocalDate().atStartOfDay(zoneId);
System.out.println(zdt); // 2015-10-18T01:00-02:00[America/Sao_Paulo]

The second program statement behaves differently than Joda-Time because it will not throw an exception but silently shifts the local time by the size of the gap in question, here one hour. This means, if you had chosen midnight, the result would be the same (namely at 1:00). If you had chosen 00:30, then the result would be 01:30. The example given above chooses noon as input.

To quote the doc for ZonedDateTime.of(…):

In most cases, there is only one valid offset for a local date-time. In the case of an overlap, when clocks are set back, there are two valid offsets. This method uses the earlier offset typically corresponding to "summer".

In the case of a gap, when clocks jump forward, there is no valid offset. Instead, the local date-time is adjusted to be later by the length of the gap. For a typical one hour daylight savings change, the local date-time will be moved one hour later into the offset typically corresponding to "summer".

A 100%-migration of all details like exception behaviour and applied DST transition strategies is not possible because both libraries are too different. But that is your guideline:

• replace DateTime by ZonedDateTime
• consider switching to LocalDate for intermediate calculations (see example)
• use explicit references to the timezone and replace DateTimeZone by ZoneId

Answer 2

You can use LocalDate to do this:

// or a similar factory method to get the date you want
LocalDateTime startOfDay = LocalDate.now().atStartOfDay();

Combines this date with the time of midnight to create a LocalDateTime at the start of this date. This returns a LocalDateTime formed from this date at the time of midnight, 00:00, at the start of this date.

https://docs.oracle.com/javase/8/docs/api/java/time/LocalDate.html#atStartOfDay--