But java.util.Date is mutable where a long isn't, and that's a problem. People can mutate a Date from under your feet if they manage to get a reference to it. This has a lot of far reaching implications, but essentially, unless you protectively copy all values of type Date you get or give, you can't trust your own internal state. You can't write a legal equals or hashCode implementation that uses a date field. The list goes on.
Don't do java.util.Date.