The 'past' that u 'go back' to might not be ur actual past. Its just another constructed universe at which the time is the same as ur past but at a different space/universe. While the space/universe that u left goes on without u.

U then, needs to go back to the time and space/universe from where u left to pick up where u have left before u go back to the 'past'.

The two variables that needs to be locked then, is the time and space, in ur 'time machine' GUI.

Of course it is....There is no harm doing it either.

Its interesting to me how we can create another plane of thinking to repair the illogical paradox of the 'grandfather paradox'. It exercise ur mind to really think our of the box and interwine the logics with the illogicals.