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.