A first-person shooter that uses Time Travel as its main gimmick, Darkest of Days is a solid game with a lot of potential that never quite gets around to really exploring it.
The game casts you as one of General Custer's cavalrymen at Little Bighorn. After the famous ambush is sprung and your fellows are cut down, an armored soldier bursts from a time portal and rescues you from certain death. Erased from history and saved from your fate, your role becomes that of a time policeman. In the actual game, this means you alternate between going back between two main time periods: the Eastern Front of World War I, and the Battle of Antietam in the American Civil War. There isn't really a far-reaching time travel aspect; those time periods are basically all you get to see.
The game's gunplay handles pretty well. You use a lot of period weaponry to maintain your disguise as a denizen of that time; in the Civil War, you're using muskets and revolvers, while in World War 1 you have more powerful bolt-action rifles. However, you also have the option of using future weapons, when you can get them; the rarity of these weapons shows their power, as an assault rifle or shotgun can make a battle much easier than it would normally be. Oddly, though, nobody ever notices you using these weapons, which makes one wonder why the Time Police bother to send you in the past with historically accurate weapons.
Your mission objectives range from the linear ("go forward, attack enemies at this location") to the open-ended ("sneak around and plant locator beacons in a German camp"). The game does a good job with atmospheric stuff; forests and cornfields are dense and give feelings of being in a tightly-enclosed area where enemy attacks can happen at any moment. The actual battles are pretty cover-intensive, too, primarily when you have primitive weapons. Using Civil War-era guns means that you have to reload very often (after every shot in the case of muskets), so finding cover is an important part of the game. There is an active reload system similar to Gears of War that allows for faster reloading (and your gun jams if you screw it up), so reloading isn't entirely tedious.
One of the only Time Travel-related gameplay points is the existence of Important People. These are otherwise normal enemies who have blue auras around them; this aura means that killing them will upset the timestream, and therefore you must shoot to wound (hitting either the arms or legs). If you avoid killing these people, then you will end up with more upgrade points to increase weapon stats. However, if you do kill those people, rival time-travelers will use the distortion of time to find you and try to kill you. Killing these enemies, though, will allow you to take their futuristic weapons for your own use. Therefore, if you're in a pinch, they can serve both as a handy armory and a nasty enemy.
The graphics are okay, but not great. They look a few years outdated, but not in such an obnoxious way as to detract from the game as a whole. The sound is pretty good in some respects (gunshot noises, environmental noises) and not so great in others (voice acting).
On the whole, Darkest of Days is a decent game - not great, not revolutionary, but pretty well made overall. The time travel thing doesn't get used as much as it ought to, but it provides some interesting levels, at least. The most annoying part of the game, though, was the fact that before and after every mission you're trapped in an unskippable cutscene where a computer screen talks at you. These sequences, reminiscent of similar ones in Half Life 2, belied the fact that the story was actually pretty one-dimensional, and mission briefings were unimportant due to the fact that your map just tells you where to go and what to do.
As a whole, Darkest of Days gets a 7/10.