This one comes via Adam and he's been using Macs for even longer than I have so maybe, just maybe, it falls into the Obscure category:

I suspect this work in other places too , like iCal, but open up Address Book and start editing someone's card. Select the Birthdate field and type "today" then hit Return.

Next try "tomorrow", "next week", "last month", "next tuesday".

Unfortunately it doesn't work for "39 years ago".