Do they just choose an arbitrary planet and use the rotation and orbit of that planet to count the days/years?
Do they use their home planet's date/time (even if it no longer exists)? Use an arbitrary date/time system based? Are there existing date/time systems that are not planet specific?
Do they use their home planet's date/time (even if it no longer exists)? Use an arbitrary date/time system based? Are there existing date/time systems that are not planet specific?
Lets see the problem from practical position. If I would be the space traveller without home. And possibly humanoid or about it. I need time units for: sleep-wake cycle, ship maintenance cycle and lifetime cycle.
Sleep-wake cycle is clear. Choose any value which is good for your race. For more information see Michel Siffre studies. In his experiments he live in cave for a six months and in result he has very different sleep-wake cycle.
Ship maintenance cycle - this is time for measuring ship fueling time or space travelling time. It is useful to measure it in tens-hundreds-thousands of sleep-wake cycles. On Earth we have moon cycle. It is not super precise in long-term but in ancient civilizations it was useful. As I remember on Easter Island time was measured in "moons".
Lifetime cycle - you can choose any big cycle events which you can see in space. It can be pulsar clock. From wikipedia: "However, for some millisecond pulsars, the regularity of pulsation is even more precise than an atomic clock". Or you can see binary star pulsation. Also pulsars and binary stars are not planet specific time measuring.