[dooglius]

There is BCT [0] which corrects for gravitational time dilation on Earth relative to the solar system, but I'm not aware of anything on a universal scale. It seems conceptually difficult to do synchronization if the communication round trip time, bounded below by the speed of light, is significant.

[0] https://en.wikipedia.org/wiki/Barycentric_Coordinate_Time

[mattkrause]

I think NTP uses some statistical tricks to get better synchronization than you'd expect over slow, laggy internet links. Possibly something like that could work.

[dooglius]

If we're talking interstellar travel, we're likely talking about spacecraft undergoing very significant acceleration and velocity. I'm not a physics expert, but from my understanding it seems that a starship would need to have a very accurate idea of its own acceleration vs reference time curve (relative to some reference frame) in order to calibrate its own ticking clock to the reference frame. For NTP, this isn't so much an issue because on Earth all the endpoints' clocks are all ticking at about the same rate.

[azernik]

It doesn't necessarily need a very good model of acceleration - you can probably get by with a) time at which a reference event is defined to have occurred, and b) distance to that event. From that and your locally-experienced acceleration you can (I think?) calculate relative speed and time drift.