Pleeeeeeeease help it's been driving me nuts!!
Xander goes back in time - I *think* it was intentional. He meets up with Spike and they get together... details very sketchy afterwards.
There's a scene where Xander visits Spike to tell him he's back (from the past) and he wants to stay together. Theres also a scene where they're standing in front of Giles and Xander debates about biting Spike as it's their normal blood sharing position, but he refrains for Giles's sake.
Sorry for shabby details. I read this ages and ages ago so it's an oldie if that helps? I know there are few time travel ones out there - I keep find the same ones over and over while looking for this one!!