That reminds me of WH40k, a fantasy universe where time/date was tracked based on when the last contact/sync [1] with earth's time has occured, with similar idea as NTP stratum.

I guess for multiple planets some astronomical recurring event could be used as sync source (say have "tick" be determined by some pulsar and adjusted for doppler shift)

* [1] https://warhammer40k.fandom.com/wiki/Imperial_Dating_System