The thing is that there exists no universal frame of reference. There is no such thing as "space coordinates 0,0,0". Those coordinates would have to be relative to some object in space.
Yes, that's kinda the crux as the relative reference of one time can and will be different in another time. Sure you can say here is a an inch cube and 10 years from now that cube will still have it's demensions relative distance to each other party upon the cube the same. But once you start adding expanding space, well, the 3 body orbit problem was bad enough, let alone at larger scales.
I still read this as a frame of reference with absolute positions.
I think the point of GP was that any time travel device you build, calculations/projections you make, will all be built around a frame of reference where the planet IS in a fixed position, as far as you are concerned. Since you don't know any other frame of reference, the machine won't either.