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

From http://codeaweso.me/2010/09/notes-on-javas-date-class/