Recently i had the pleasure of watching the creators of Achron speak about Time Travel in games (and other mediums). They are local here in NC and come straight out of NCSU. Achron is an RTS game that uses a Time Travel mechanic in a multiplayer setting. Players have a limited resource to spend on going back in time to alter it (sending units and change events) which then propogates those changes as waves moving ~2x's real-time to the present. I reccomend watching the video on it :).
Sadly there are no 1982 Deloreon units..... or barn buildings, at this time. :)