We measure time based on local astronomical events. One year is how long it takes the Earth to orbit the sun. One day is how long it takes the Earth to rotate on its axis. But what would happen in a galactic society? Each planet would have their own system for measuring time, but the greater society would need a standard to follow. Different species would have different needs, since circadian rhythms are determined by day length. Having different species on a starship would be difficult since a species that has a circadian rhythm of 30 human hours would suffer different problems if the ship were kept on a 24 hour clock, and species that have a circadian rhythm of 18 hours would have have different problems. And then you have cultural and legal issues, such as age of majority. I would imagine that Starfleet would require each species to certify adulthood status on applicants, but if one species matures after 3 Earth years and others are still juveniles at 100 Earth years, it might make things confusing. How would you handle this?