You can travel forward in time by traveling near the speed of light, but going backwards in time is a much more complicated issue. It is generally accepted by theoretical physicists that IF backwards time travel is a possibility, you create (or at least travel to) a parallel universe, otherwise you (and everything else) exists in a world riddled with paradoxes.
Think about if you traveled backwards in time 5 minutes to the exact same spot. If you were to kill your other self before they could travel backwards 5 minutes, you would have never been able to travel backwards. It is like the scene in back to the future where marty starts to disappear because his mom isn't with his dad anymore, but this is a very simplified portrayal of the problem. If you were to disturb where your dad was by even 1 nanosecond, the odds that your parents would have sex at the same place, at the same time, in the same position, allowing the same sperm out of 300 million of them to reach the egg, are like 10000000000000000000 to 1. Thus you would have never existed as you do (did/ will/ I dont know) to travel back in time and make the change in the first place.
As you can see, paradox.
Not to mention that if you were to travel back in time, any change that you made would have to instantaneously be reflected in the "present" that you just left. You could travel back 100,000,000 years and kill the early humans, and the second you went back in time anybody watching your delorean would instantly disappear.
Thus, the only way around these paradoxes is that the only way that backwards time travel is possible is to create or travel to a parallel universe, so even if they traveled back to February 18th, 2012, you have no chance of seeing them. They could come meet you, but you can never meet them.
Its heavy, doc.
