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All the (human, duh) calendars I'm aware of, are based on Earth's rotation around itself and the sun and/or the moon's around us.
For the Gregorian calendar, for example, the day is heavily inspired by the Earth rotating around its own axis. The year is based on the Earth going round and round around the star. As of the epoch year, it's supposed to be some prophet's birth year (how precise is this date, is out of scope on this question).
This makes sense for us, since we always lived on Earth. But for societies who will live, for generations, on giant spaceships roaming indefinitely in the interstellar space, this is not relevant.
For these kind of societies, what would an optimal calendar look like?
this system must be usable for communications between ships. For this, note that it may take a thousand years for a message to get to its destination. (Don't ask me what would such a message be. I don't know either!);
this system must also be usable for communications between ships and planetary bases, each of which is on its own weird star-system. If this seems too hard, we can just consider Earth-based settlements.