Converting between java.time.LocalDateTime and java.util.Date

Short answer:

Date in = new Date();
LocalDateTime ldt = LocalDateTime.ofInstant(in.toInstant(), ZoneId.systemDefault());
Date out = Date.from(ldt.atZone(ZoneId.systemDefault()).toInstant());

Explanation: (based on this question about LocalDate)

Despite its name, java.util.Date represents an instant on the time-line, not a "date". The actual data stored within the object is a long count of milliseconds since 1970-01-01T00:00Z (midnight at the start of 1970 GMT/UTC).

The equivalent class to java.util.Date in JSR-310 is Instant, thus there are convenient methods to provide the conversion to and fro:

Date input = new Date();
Instant instant = input.toInstant();
Date output = Date.from(instant);

A java.util.Date instance has no concept of time-zone. This might seem strange if you call toString() on a java.util.Date, because the toString is relative to a time-zone. However that method actually uses Java's default time-zone on the fly to provide the string. The time-zone is not part of the actual state of java.util.Date.

An Instant also does not contain any information about the time-zone. Thus, to convert from an Instant to a local date-time it is necessary to specify a time-zone. This might be the default zone - ZoneId.systemDefault() - or it might be a time-zone that your application controls, such as a time-zone from user preferences. LocalDateTime has a convenient factory method that takes both the instant and time-zone:

Date in = new Date();
LocalDateTime ldt = LocalDateTime.ofInstant(in.toInstant(), ZoneId.systemDefault());

In reverse, the LocalDateTime the time-zone is specified by calling the atZone(ZoneId) method. The ZonedDateTime can then be converted directly to an Instant:

LocalDateTime ldt = ...
ZonedDateTime zdt = ldt.atZone(ZoneId.systemDefault());
Date output = Date.from(zdt.toInstant());

Note that the conversion from LocalDateTime to ZonedDateTime has the potential to introduce unexpected behaviour. This is because not every local date-time exists due to Daylight Saving Time. In autumn/fall, there is an overlap in the local time-line where the same local date-time occurs twice. In spring, there is a gap, where an hour disappears. See the Javadoc of atZone(ZoneId) for more the definition of what the conversion will do.

Summary, if you round-trip a java.util.Date to a LocalDateTime and back to a java.util.Date you may end up with a different instant due to Daylight Saving Time.

Additional info: There is another difference that will affect very old dates. java.util.Date uses a calendar that changes at October 15, 1582, with dates before that using the Julian calendar instead of the Gregorian one. By contrast, java.time.* uses the ISO calendar system (equivalent to the Gregorian) for all time. In most use cases, the ISO calendar system is what you want, but you may see odd effects when comparing dates before year 1582.

Much more convenient way if you are sure you need a default timezone :

Date d = java.sql.Timestamp.valueOf( myLocalDateTime );

Here is what I came up with ( and like all Date Time conundrums it is probably going to be disproved based on some weird timezone-leapyear-daylight adjustment :D )

Round-tripping: Date <<->> LocalDateTime

Given: Date date = [some date]

(1) LocalDateTime << Instant<< Date

    Instant instant = Instant.ofEpochMilli(date.getTime());
    LocalDateTime ldt = LocalDateTime.ofInstant(instant, ZoneOffset.UTC);

(2) Date << Instant << LocalDateTime

    Instant instant = ldt.toInstant(ZoneOffset.UTC);
    Date date = Date.from(instant);

Example:

Given:

Date date = new Date();
System.out.println(date + " long: " + date.getTime());

(1) LocalDateTime << Instant<< Date:

Create Instant from Date:

Instant instant = Instant.ofEpochMilli(date.getTime());
System.out.println("Instant from Date:\n" + instant);

Create Date from Instant (not necessary,but for illustration):

date = Date.from(instant);
System.out.println("Date from Instant:\n" + date + " long: " + date.getTime());

Create LocalDateTime from Instant

LocalDateTime ldt = LocalDateTime.ofInstant(instant, ZoneOffset.UTC);
System.out.println("LocalDateTime from Instant:\n" + ldt);

(2) Date << Instant << LocalDateTime

Create Instant from LocalDateTime:

instant = ldt.toInstant(ZoneOffset.UTC);
System.out.println("Instant from LocalDateTime:\n" + instant);

Create Date from Instant:

date = Date.from(instant);
System.out.println("Date from Instant:\n" + date + " long: " + date.getTime());

The output is:

Fri Nov 01 07:13:04 PDT 2013 long: 1383315184574

Instant from Date:
2013-11-01T14:13:04.574Z

Date from Instant:
Fri Nov 01 07:13:04 PDT 2013 long: 1383315184574

LocalDateTime from Instant:
2013-11-01T14:13:04.574

Instant from LocalDateTime:
2013-11-01T14:13:04.574Z

Date from Instant:
Fri Nov 01 07:13:04 PDT 2013 long: 1383315184574