Joda time's DateTime converted to java.util.Date strange issue

Question

I ran into a strange issue. Here is a snippet of code that describes it:

DateTimeZone dtz = DateTimeZone.forOffsetHours(0);

DateTime dt = new DateTime(dtz);

System.out.println(dt);
System.out.println(dt.toDate());

the output is:

2012-02-29T17:24:39.055Z
Wed Feb 29 19:24:39 EET 2012

I'm located UTC+2, but this action is supposed to create a java.util.Date object which is initialized for UTC time. What am I missing?

Answer 1

Date doesn't know about a time zone at all - it only represents an instant in time (like Joda Time's Instant type). It's just a number of milliseconds since the Unix epoch. When you call Date.toString(), it always uses the system local time zone for converting that into a readable text form.

So there's nothing wrong here - just an expectations failure over either the meaning of java.util.Date or its toString() behaviour, or both.

(As an aside, prefer DateTimeZone.UTC over creating your own.)

Answer 2

To get a JDK Date that matches Joda's DateTimeconvert to LocalDateTimefirst.

As explained in the other answers, the time in milliseconds does not change depending on the timezone:

DateTime local = DateTime.now()
Date localJDK = local.toDate()
assert localJDK.getTime() == local.toInstant().getMillis()

DateTime differentTimeZone = DateTime.now(DateTimeZone.forID('America/Chicago'))
Date localJDK2 = differentTimeZone.toDate()
assert differentTimeZone.toInstant().getMillis() == localJDK2.getTime()
assert localJDK.getTime() == localJDK2.getTime()

Converting a LocalDateTime to Date will change that:

Date differentTimeZoneJDK = differentTimeZone.toLocalDateTime().toDate()
assert localJDK.getTime() != differentTimeZoneJDK.getTime()