Notes on Java’s Date class

Every so often I get myself confused about Java’s often-obtuse handling of dates and timezones. So, for my future self’s benefit, here are some reminder notes…

Please leave a comment if you have corrections or additional tips! (And, preemptively, yes, some day I’ll give Joda Time a try.)

Dates do not have timezones; they reflect UTC.
Date.toString() returns a String representation based on the local machine’s default timezone. This makes people think that Dates have timezones.
You cannot convert a Date from one timezone to another.

Calendar c = Calendar.getInstance();
c.setTime(new Date());
c.setTimeZone(TimeZone.getTimeZone("UTC"));
Date utc = c.getTime();
c.setTimeZone(TimeZone.getTimeZone("America/Phoenix"));
Date mst = c.getTime();
assert !utc.equals(mst);  //throws AssertionError

However, you can use DateFormat to affect the String representation of a Date, including the timezone.

Date date = new Date();
SimpleDateFormat utcFormat = new SimpleDateFormat("ddHHmm");
SimpleDateFormat mstFormat = new SimpleDateFormat("ddHHmm");
utcFormat.setTimeZone(TimeZone.getTimeZone("UTC"));
mstFormat.setTimeZone(TimeZone.getTimeZone("America/Phoenix"));
String utc = utcFormat.format(date);
String mst = mstFormat.format(date);
assert !utc.equals(mst);  //should not throw AssertionError

Mike Christianson

Notes on Java’s Date class

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