Darkest of Days is a game I hadn't even heard about until recently, and I get the feeling I'm not alone. Sure, it's a first-person shooter, but the premise itself is pretty neat, and Phantom EFX seems to be taking a different approach to the genre.
Someone (or something) is altering the past by placing highly important historical figures in situations which they cannot possibly survive in. It's -- you guessed it -- up to you to stop whatever sinister forces are at work before someone inadvertently kills a bug which causes the present to be a horrible, lifeless wasteland.
And since you're going back in time, yes, high-tech weapons will allow you to kick some major ass. That said, from the look of this video, you should be able to use the standard weapons of whichever time period you are in as well.
Oh, and the release date? It's going to be September 8 for both PC and Xbox 360.