Re: Paging Mr. Einstein...
I'm wrong about that: it can in fact do a little better. Since it can measure the non-inertialness (linear acceleration, rotation) of its frame, it could if it wanted to correct for that, assuming it's in flat spacetime. So for instance in the twin 'paradox' a if the twin who gets accelerated measures their acceleration & integrates it up suitably they can predict the time told by a clock the other twin is carrying.
What it can't do is deal with curvature, since it can't measure that (well, not easily, and not unless its very large, or the curvature is very high indeed).
Biting the hand that feeds IT
