Cue "Born Yesterday."
"That's funny," says I. "December 14th 1901 works fine, but the 13th is no good at all, nor anything earlier than that!"
So if you're 109 years old and join Facebook, they won't let you show your birthday! Same goes for anyone older (I bet that 122-year-old is pissed). Seems pretty ageist, don't you think? And what's so bad about December 13th, eh? I mean, Ted Nugent was born on December 13th, but you can't hold that against the day itself...
Hang on, what's December 13th in Unix time?
$ date --date '1901-12-13' +%s
Well, I say! That's just a bit past -2147483648, which is... ah, that's right, -(2^31), or the lowest number that can be represented by a signed 32 bit integer. Guess we know how Facebook is storing their dates, eh? Too bad they didn't bother limiting the options on their date picker to match their technological limitations.
They must have some validation in place to prevent you from having a birthday in the future, so when the integer rolls over to a positive number, it just gets cut down to the current day. Pity, really. I was looking forward to being born on January 18th, 2038.