Here my physics knowledge gets a little sketchy, but if we have an accurate clock broadcasting on a known frequency somewhere far away we should be able to at l

Here my physics knowledge gets a little sketchy, but if we have an accurate clock broadcasting on a known frequency somewhere far away we should be able to at least measure the drift between it and a local clock to arbitrary precision. With ~zero drift we can measure the distance and relative velocity very accurately with round-trip timing and Doppler measurements, and from that measure the clock offset as accurately as we can measure the distance. I think, but could be wrong, that we'd always be measuring the spacetime interval, not the euclidean distance in space, but that is probably what we actually want once we start caring about relativistic timekeeping.