Is there such a thing as a "zero date"? I don't think so. Your code also may be giving you unpredictable results because you are only setting the time portion of the date. When I run it I get 18000000, which is Thu Jan 01 00:00:00 EST 1970. I'd stick to using DateFormat for producing output since Date has some funky internal representation (years are 1900-indexed meaning this year is 104 and months are 0-indexed).
Both Date and Calendar represent real dates, that is, a point in time having a year, month, day and so on. I don't think you can use either class to get a difference (i.e. dateInstance1 - dateInstance2 = elapsedTime). If you look at the documentation for java.util.Calendar, you'll see that when not set, its fields default to certain values in order to prevent ambiguous class states. You'd do better to compare the individual fields (i.e. dateInstance.getDay() - dateInstance2.getDay() = elapsedDays) or, if you just need to know if a certain deadline has expired, use Calendar.add() to move a date, say 30 days ahead, and use Calendar.before() or Calendar.after() to do bounds checking.
Joined: Apr 29, 2004
But what if I work with periods that are longer than, say, 30 days? If I want to know the number of days between 2004-01-01 and 2004-03-01, then I run into problems with Calendar#get(int) (e.g. leap years etc)... It seems that I didn't get the hang of it.