ETA - Have got it, thanks to amybnnyc for the memory jog.
OK - I don't know how anally retentive the questions will be on The Einstein Factor, and I'm probably overstudying, but this has been bugging me. In the Season 1 episode I Robot - You Jane, Buffy's DOB flashes on a computer screen twice, in the first one she's listed as a Sophomore and her birthdate is 10/24/80 (24 Oct 1980), and then the next one she's listed as a Senior and her birthdate is 05/06/79. Obviously dates were retro canoned, or whatever it's called, after Season 1. In the later seasons I thought her birthday usually aired around Feb in the States, and that was when it was supposed to be. OK I've checked these are the airdates for Buffy's birthday celebrations that I can remember, Surprise 19 Jan 1998, Helpless 19 Feb 1999, A New Man 25 Jan 2000, Blood Ties 06 Feb 2001, Older and Far Away 12 Feb 2002.
So here's my question - does anyone know if her DOB has ever been mentioned anywhere else, I'm sure it has, but I can't think of it.