 Buffys birthday was celebrated in the episode first aired on or  near 19 January each year. In this episode, Malcolm brings up  Buffys school records on his computer. In the first view, we see  that Buffy was born on 24 October 1980, and that she is a sophomore.  The second time we see Buffys records, her birthday is 6 May 1979,  and she is a senior. (She is also shown with two different GPAs, one  of them excellent.) Joss Whedon (qv) finally decreed Buffys  birthday to be 19 January 1981.