I always wondered if it was a numbers thing for my dd5. I've mentioned in previous posts that she was quite obsessed with numbers -- to the point where she refused to wear a shirt unless it had a number on it for the entire year she was two (bless my mother's heart, she sewed numbers on them all). She has always noticed numbers, like addresses, where ever we go. That's not that odd in itself. The strange thing to me is that she retains these numbers for many months or years and then recalls them effortlessly.
It's almost as if her memories have a date stamp. Except it's not just the numbered date, she often remembers the day of the week as well. But I don't think she could compute a day of the week simply by knowing the year. I think her date recall has to do more with her own memories.
I've grown quite accustomed to her quirky ways. But this may be the one thing she does that always catches me off-guard and makes me think, "That's not normal, is it?"