In the episode "Halloween" when Buffy and Willow are sitting in the bathroom of the school reading one of the watcher's diaries they come across an entry from 1775 and Willow says "Angel was eighteen, and still human." But it has been established time and time again that Angel was born in 1727 and died in 1753. If you check the math you can see that the entry from 1775 was written 22 years AFTER angel died. And we always thought Willow was so good at math.