Java 8 has a completely new API for date and time. One of the most useful classes in this API is LocalDateTime
, for holding a timezone-independent date-with-time value.
There are probably millions of lines of code using the legacy class java.util.Date
for this purpose. As such, when interfacing old and new code there will be a need for converting between the two. As there seems to be no direct methods for accomplishing this, how can it be done?
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 from:
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.