could it be
1880 by bittenandstaked? There is a thread here for this, but here is the swayback machine link.
http://web.archive.org/web/20070714191600/http://www.talesoftheslayer.com/1880/188001 .php. Summary: Travelling through time is the last thing Buffy expects to do when she gives chase to a man who has stolen something from the Magic Box. Finding herself in nineteenth century London, there's only one person that Buffy can think of who could help her get home, Spike, or at least his human counterpart, William. The only question is whether Spike is still William at this moment in time or whether it's too late and she'll be stuck in 1880 forever. And even if she finds William, will he be able to help her back to her time and back to the vampire version of himself that she discovers she has feelings for?