In interstellar civilisations there is no obvious way to measure the passage of time in the same way as there is for a species confined to a single planet. As such, different cultures use very different systems and timescales.
Alliance Time
The Reunification Alliance measures time in Standards, units of measurement heavily based on Earth time. This system is also commonly used on former Alliance colonies, including those that broke away during the Crisis of Faith. The exact progression of Standard Time is tracked by the Alliance Central Calendar, which other worlds sync with using Otherlight communications.
Standard Time units are roughly based on Terran rotational and orbital periods, but adjusted to more round numbers. The Standard Week is typically used as the main measure of short to moderate periods of time.
Alliance Standard Units:
Standard Day - 24 Hours
Standard Week - 10 Days
Standard Month - 4 Weeks
Standard Year - 9.5 Months
The nine full months in each year are referred to simply as Firstmonth through Ninemonth, and written numerically as 01-09. The half-months that straddle the new year are collectively called Newmonth (written numerically as ‘NU’), so the new Alliance year begins 21/NU/xxxx.
Most worlds using Standard Time will also keep track of their orbital days and years. This is done using a local calendar, with the lengths of the days and year corresponding to the appropriate astronomical measurements. Weeks are typically retained at ten days and months at forty, though for some worlds with particularly short orbital periods one or both of these may be redundant. Extra or missing days are accounted for by extending or shortening the final week/month of the year correspondingly, with sub-day differences accumulated into leap years. The number of years passed is not tracked by local calendars, with the new year simply denoting a need to reset the count of days/weeks/months. As such, even the local date exists in the context of Standard Years, regardless of how they actually sync up.
Which date is most commonly used in daily life depends on the customs and physical realities of each individual planet. The primary consideration is maintaining a logical passage of time that more-or-less lines up with the patterns of local life. Whichever date is chosen as the ‘primary’ date is written first, with the ‘secondary’ date appended to it brackets and the difference between them noted in superscript. Spaceports always use the Standard Date as the primary date.