He didn't remember her birthday.
He remembers celebrating them. He sent her a smutty card for her 18th, not a cutesy-kittens one. Buffy bristled reflexively on seeing it, but understood: Dawn’s a grown-up now. He remembers her birthdays. He just couldn't remember her birthday.
He’d known the date since she was ten. And that's the problem: those memories weren't real. They faded when magic left. Dissolved away like crystals, leaving little holes in his brain. Facts he can't remember. Things he doesn't know he's forgotten.
He's written her name on a piece of paper in his wallet.
Just in case.