Time is the passage of events. What is an event? An event is a change of state. When an atom acquires an electron, when a subatomic particle changes its spin, or direction, or velocity, when you drop your baloney sandwich on the floor--all of the above are events. So, time is our way of describing a changing universe.

What, then, is time travel? Time travel (it isn't really "travel" but I'll use it for lack of a better word) would require setting the universe to the exact desired state.

For example, if I wanted to travel to 12:23pm on June 14, 1659, I would have to: (1) know the state of all matter and energy in the universe at that particular instant, and (2) have the ability to cause the universe to revert to that state. This is no small task.

And that is why time travel is impossible.