Probably easier to just muck with the HP timeline, since there's some uncertainty there.
On the other hand it's way easy to put Giles at school with Snape, James, Lily, etc., as Pointy horns or black hats did.
Fitting into existing timelines is tricky. In one of his prefaces, I forget which, Patrick O'Brian writes thanks to bad planning he ended up writing the year 1812 three times over. And if the great PoB has trouble, us lowly fanfic writers can be forgiven for struggling. I am having big trouble with the summer of 1999. I want way too much to happen during it. I think I'm just gonna punt and cram it all in and look vaguely off into the distance when questioned.