time is subtly hard to get right
Yeah definitely hard, I did some research upon the NTP protocol and the work of Mills years ago. To my suprise (then) the NTP uses statistics to handle latencies of the network and the server. For me that looked like "the first signs of some variant of the Heisenberg principle" 
I recall that Mills started to investigate NTP for intersolar communication (one time ref per solar system, NASA) to synchronize satellite data. Depending on the relative speed of things he needed to compensate for relativistic effects.
Definitely hard.