This kind of time-travel is not new in film, of course, though I don't recall it being used in a horror slasher before. But as a story device, there are examples aplenty in science fiction, comedy, drama... always showing how even the tiniest details can have large (and largely unexpected) results, and that even foreknowledge only gets you so far in doing better. There are anime, old black and white films, movies where people shift only once, or many times...
Which film did it best though? Let's make that our question of the week. Chime in, in the comments below, and HAVE YOUR SAY!