It's been like that with pretty much every chick I've been with. Usually after a year and a half or so, you both start forgetting how many months and whatnot. I mainly can't remember because I never even know what day it is.
Edit:
Ironically, now that Ithink of it, it's the 24th already and it's our monthsary.

I can never for the life of me remember how many months it is.