Your right, time is a unit of measurement, not an entity. However, time travel itself is possible. I will try and go into some specifics, but I really don't have the time (no I wasn't trying to be funny). So I'll give you what I know off the top of my head.
Satellites such as the global positioning satellites need their eternal clocks periodically adjusted because they all go faster than the ones on earth.
(I show my work) http://en.wikipedia.org/wiki/Time_dilation
"For GPS satellites, GR predicts that the atomic clocks at GPS orbital altitudes will tick faster by about 45,900 ns/day because they are in a weaker gravitational field than atomic clocks on Earth's surface. Special Relativity (SR) predicts that atomic clocks moving at GPS orbital speeds will tick slower by about 7,200 ns/day than stationary ground clocks. Rather than have clocks with such large rate differences, the satellite clocks are reset in rate before launch to compensate for these predicted effects. In practice, simply changing the international definition of the number of atomic transitions that constitute a one-second interval accomplishes this goal. Therefore, we observe the clocks running at their offset rates before launch. Then we observe the clocks running after launch and compare their rates with the predictions of relativity, both GR and SR combined. If the predictions are right, we should see the clocks run again at nearly the same rates as ground clocks, despite using an offset definition for the length of one second."
This is in the most simplistic of terms caused by gravity. You may of heard the documentary say "Space Time" as in space and time are virtually the same thing. This is right, but I can understand the confusion. You see, objects such as the Earth actually bend space around it. This is proven through good old Einstein. Since space and time are one it actually bends time when you introduce a large gravity affecting object.
This is the best picture I could find that represents this
Now as you get closer to the Earth time slows down compared to the time on earth.
You also have to remember that speed also is a factor of time. As a body moves closer and closer to the speed of light time on and in it slows down. Once again more Einstein. Remember that the Earth isn't just moving on its own, its moving around a solar system that in itself is moving around a galaxy. So in reality the Earth is moving at 550,000 miles per hour or about 152.7 miles a second. The speed of light is 186,000 miles per second. Now that may seem like a huge difference, but it is still enough to affect time around the Earth. Now I am unaware if this last part has been proven or not. I do believe scientist are currently trying to study black holes to prove this theory as they tend to pull things in at around the speed of light.
Now you may be thinking, bullshit. Astronauts go up in space all the time. Spending months at a time in space yet visually their age remains the same. Well you are correct. While indeed they have "gone in time" the amount of it is tiny, only a few milliseconds if even.
Hope this helps
