propose simple valid date test for ruby leap year implementation

I realize that our spider senses tell us to turn up our "code smell" sensitivities when we see `begin..rescue`; however, since ruby does not have another way to say (unless I've missed it), "hey, is this set of numbers a valid date", I've opted for `begin..rescue` for concisness. My rationale is that I'll sacrifice whatever performance penalty (whether real or pedantic) for readability unless it becomes an issue.