You can only go back in time using a looking glass. You cannot physically go back in time.

It's like when you are writing a file on a DVD. Once you have written it, you cannot erase it. You can only view the pass by using the looking glass.
[link to www.projectcamelot.org]

However you can go to the future. Since time is like a cycle and you can only go forward, if you want to go back in time you will have to make a big round into the future and after the end of time and when a new time start again. Of course by then the "pass" will not be the same as you thought it is. That's the real meaning of going back in time.