[AndrewC]

and that's a time machine.

Just how would this thing work? If you "go back" in time, the earth has moved, space has expanded etc. etc. So the "way-back" machine may put you somewhere you don't want to be at a time of your choice. (i.e. it must be a time-space machine)

[VadeRetro]

Does it put you where the earth is now in space or where it was back then? Does it mess up history until a version of history is created in which no time machine was never invented? (Premise of an SF novel I've otherwise largely forgotten.)

[PatrickHenry]

If you "go back" in time, the earth has moved ...

I know, I know.