If you have a choice, it would probably be better to use
System.currentTimeMillis()
final long before = System.currentTimeMillis();
doStuff();
final long after = System.currentTimeMillis();
final long duration = after - before;

However, if you only have access to the Date objects:
final Date dateBefore = new Date();
doStuff();
final Date dateAfter = new Date();
final long duration = dateAfter.getTime() - dateBefore.getTime();