Hand-wavy ‘the machine follows the weighted average of the immediately adjacent matter, thru time”
Similarly a hand-wavey "the time portal is sparked through the physical quantum entanglement of the same particle through different points in time"
I just finished reading All Our Wrong Todays by Elan Mastai, which deals with this 'Space Problem' as part of it's plot. There's also the issue of what happens if the space you end up in is already occupied... ouch.
I would expect to find myself in empty space and prepare appropriately. A civilization that can produce a time machine could with less difficulty produce a space vessel with faster-than-or-near light speed travel.
This is why the TARDIS from Doctor Who is called Time and Relative Dimensions In Space.
All sci-fi movies involving time travel are now ruined.
Er..more ruined. Ruined-er?
As Douglas Adams pointed out, the hardest part of time travel is correct grammar.
that's presuming you are traveling backward instantaneously, or very quickly. if you moved more gradually, like the way we the way we move through space, presumably you could stay pinned to earth ...