Out of Time: a Time Travel Novel

In the 2150's, a scientist discovers the means to time travel and he wants to travel into the past. The US government wants him to find out if history can be changed. The theme being to make right what once went wrong. When the time travelers end up back in their present time, they find things have changed, but not necessarily for the better. Will they return to the point in time where everything changed, or will they decide they've messed with history too much? Find out in this time travel story.