I know that system clock drift can be bounded to milliseconds using an atomic clock or GPS receiver, but I didn't know whether we could overcome synchronization difficulties when speed-of-light differences are significant. I thought we could package some coordinate reference system, directional travel metadata (I'm traveling 0.000002c this way), and current system clock time, and synchronize that way.

Fantasy use case: Asteroid mining and traffic control. If you want to boost prepared asteroids to an orbit closer to Mars and you had a space station / spaceships to watch out for, different mining companies might want a clock protocol, a request buffer, and a map instead of synchronously planning, timing, and verifying each and every asteroid orbit change in a central location ("What do you mean you were using Earth time and not correcting for relativity??").