Time travel is the concept of moving backwards and/or forwards to different points in time, in a manner analogous to moving through space, and different from the normal "flow" of time to an earthbound observer. In this view, all points in time (including future times) "persist" in some way. Time travel has been a plot device in fiction since the 19th century. Traveling backwards in time has never been verified, presents enormous theoretic problems, and consequently is impossible.
A central problem with time travel to the past is the violation of causality; should an effect precede its cause, it would give rise to the possibility of a temporal paradox.
More, If Andy Basiago went back in time to Gettysburg once, he also went back infinite times. In fact he was in that time-place point before his own travel.
He was there jumping back in time before the existence of CIA time machine. This is impossible, of course.
In other words Basiago was going back in time before he was doing it, and this phenomenon repeats ad infinitum.
If there would be travel in time, history would not be an unchangeable constant, since any change made by a hypothetical time traveler would change completely our present times.
Time is a dimension in which events can be ordered from the past through the present into the future, and perhaps more important, Time is the measure of durations of events and the intervals between them.
A moment in time does not in any way remains in the past, waiting for us to "go" there. This idea is not only impossible but ridiculous.