Relativity says it’s impossible for two observers to agree on things like simultaneity, distance, duration, etc. Without reference to astronomical landmarks (observed within a given proximity of the solar system, at low speed) how could anyone agree on the meanings of time intervals?
I believe the cosmic microwave background provides a reasonably universal rest frame to anchor your definitions of time. It should at the very least be a lot less local than using the rest frame of a particular solar system.
