You can’t go back in time even if you could find a way to travel faster than the speed of light. Time dilation doesn’t work that way, because you can’t dilate something less than zero.
Superliminal velocities do not involve time dilation, which is a difference between an observer's proper time and the time he measures for another observer either in a gravitational field or in relative motion to his frame of reference. Both frames of reference are inside the light cone.
Time dilation only occurs as objects approach the speed of light from below. It does not apply to hypothetical superluminal objects. These objects are "spacelike" and outside of the light cone. When they (hypothetically) decelerate to speeds actually allowed in our universe they would arrive before they left.
Don't doubt me. I've taught university physics courses in relativity.
Wikipedia's discussion is actually accurate and accessible to laymen: http://en.wikipedia.org/wiki/Light_cone