So I poke a hook into spacetime at the beginning of the Unix epoch.
Planetary rotation immediately disambiguates that location from its current equivalent position at the current second.
And orbiting around the moon earth center of gravity - axial tilt?
Axal drift due to water and ice shifts
Orbiting around a star
That star's orbit around the galactic center
That star's bobbing through the galactic plane
The movement of the galaxy
That's before relativistic effects
...what am I missing
Yet another one user instance
That is to say, a Spacetimestamp of usable resolution might be
8.8 * 10^26 m across for the visible universe... let's give it 128bits for micron accuracy. Just because 96 bits isn't enough for millimeter accuracy.
That's 448 bits for 3.1 dimensions. Then you need to start adding on details to have any indication of what the relevant location may be now.