Summary: How do you maintain standards of time, across cosmic distances?
To bridge the gap between Sol and Centauri, the first starships left a series of beacons in the void. These were strategically positioned between the outermost distances of the Sol system and others, transmitting messages by radio, laser, and neutrino bands.
As Terran civilization spread ever outwards, a communication network flare outwards from Sol. Each one had an atomic clock, relating to the prior node in the network. While there were gaps and outages, timestamps could be interpolated in a fairly reliable way.
Across history, standardization of timekeeping was a perquisite for further political unity. The cosmic future was no different, although local conditions changed before responses came from the center. As such, the network became a method of preserving history, as well as cosmic communication.