@Efi Oh frick how could I forget about atomic time lol (Tho that's affected by earth's mass per relativity, so maybe one located somewhere in solar orbit would be best?)

And yeah, tho months get kinda hard when you have dozens of moons ^^ I suppose you could chose one arbitrarily? (Or just the first to be colonized).

For days it might make sense to chose a global amount of hours to use everywhere, because a ganymedean day is 172 hours! Compare that to titanian days, which are 16 days! Compared to Martian days, which are a 24 hours and 40 minutes!