Rules:
*You can teleport into and out of it at will
*It has a couple of plug sockets and can connect to internet from the region you teleported in from
*You can take objects and people with you
*As already stated, it is 3m^3. The walls are plain plaster with a light in the middle of the ceiling. The pocket dimension is topologically toroidal, so if there weren’t walls and a ceiling/floor (which you can actually destroy) you would loop if you went more than 3m in any direction. Gravity, then, is artificial and can be altered to anywhere from 0 to 2g from a dial on the wall.
Wouldn’t the same logic apply to the planet moving?
I think it was the Hitchhikers books that made that joke in relation to people inventing time travel. They travel to some other point in time and die in space because the earth isn’t in that spot any more.
All the real theoretical kinds of time travel involve a physical path you have to move along with a specific start and end point, because yeah, otherwise the frame of reference would be ambiguous.