There is evidence that time travel is possible, or that time can be ignored by entangled particles, and it is these two vaguely related ideas which I have combined into my own pseudo-scientific theory called "Temporal Entanglement".
If we make time an essential element of space--not just time, but that time which happened in that space--then time is unique to location. It's possible the location could be as small as a photon, or maybe within our own minds.
A time machine would have to tap into the entangled properties of space/time, or it wouldn't work, like that thing that doctor uses. The good news is the machine wouldn't go careening off into solid rock or unexpected vacuum because it moved in time, but not in space.
I'm sure I had a better point to make when I started writing this. . . .