When considering the possibility of traveling at relativistic speeds, the theory of time dilation tends to become the problem of applying this accomplishment. The theory that at relativistic speeds time slows down and ultimately stops as your achieve the speed of light.

However, one must take into consideration special relativity. The speed of light is constant, yes. But only relative it's source. The light traveling towards an observer that it not moving relative the light source, moves towards the observer at 299,792,458 m/s("c"). However, if the observer is traveling towards the light source at the speed of light, then the light would be traveling at 2c (599,584,916 m/s) relative the observer.

If you travel away from a clock at "c" for 3 hours at 3pm, you will observe, upon reaching your destination, 3pm on the clock whereas your watch will read 6pm. This is because you and the light from the clock both left the clock at 3pm traveling the same speed. The light that is reaching you from your watch is traveling only about a foot and a half maybe, but from the clock it's traveling 3 light hours.

Here's the big kicker. You know have 3 hours of light between you and the clock. Traveling back towards the clock at "c" will cause you observe the light at a much accelerated rate. Plus you'll be observing for 3 hours. So that's six hours of light catching up to you in 3 hours. Your watch and the clock have now caught up with each other. It's 9pm.