I've found this game (well, I've known about it for a while actually) which is a time-travelling strategy game. You don't choose a time-period, like other rts' you can actually go back along your own timeline and correct any mistakes you made, and send units back in time to fight alongside their past counterparts. It's still in alpha at the moment but it is promising (I've bought and downloaded it).
Here's the link: [You must be registered and logged in to see this link.]
Here's the link: [You must be registered and logged in to see this link.]