part one
I see time as this:
We travel through 'space-time', everyone understands the space part, but you should realise that we travel exactly at the same way through time.
So we travel to a combination of space and time, with time as a fourth dimension. This four dimensions together is 'space-time'.
Let's say we travel through this 'space-time' with a kind of speed, this speed is what we normally think of when we talk about time, because we don't know another speed.
conclusion part 1:
when we say speed of light = c
we can say our speed through space-time = 'our' time (time as most people see it; relative)
part two
concluding to part one:
We never can travel in time the way most people see it; in one moment from 2001 to 2050 or to 1970 (when Jim Morrison was still living and you could see one of his performances).
But we can slow down our speed through our space-time.
When we can come in the vicinity of a black hole, time slows down. That's concluded by our lovely scientist Einstein in his theory of relativity. Probably there must be something like this to accelerate our speed (in fact time goes faster when you're in outer space then when you're on earth, but there is just a very small difference).
So we can change the speed. But can we turn around our direction of traveling? No, I can't think of a way to do that...(ideas are very welcome offcourse)
conclusion part 2:
Can we 'travel' through 'space-time' ??? no
Can we change our change our 'speed' through 'space-time'??? yes
(everything clear??)