Marking Time
If you thought time zones were tough, realize that every planet's got its own orbit and rotation, and let's not talk about the weird ones that orbit gas giants or whatall...then there's the space habitats, and it just gets all kinds of confusing.
When you're in a place like that, use their time.
Everywhere else, there's Universal Time.
The Galactic Time Code is a standard time measuring system, that can be independently synchronized through astronomical observations. The primary means is to monitor the gravity waves coming from the galactic core. The rapid movement of heavy giant stars around the supermassive black hole at the center produces a chaotic pattern of gravity waves that never repeats, but can be mathematically predicted. With a few minutes of observation and a powerful computer, you can figure out exactly when you are, at least within a few hundred thousand years or so.
This method requires gravity wave detection, which isn't exactly easy to put together on a budget. A secondary method involves measuring the frequency of known pulsars. Every pulsar in the galaxy has been meticulously mapped out, and even with rudimentary, pre-industrial technology, you can get a decent read on some nearby pulsars, and not only figure out where you are in the galaxy, but when. That's because all pulsars spin at known rates, but that rate decreases ever so slightly over time. So even if you fall through a wormhole and end up a million years in the future, half a galaxy away, this method could still tell you where and when you are. Without advanced technology, the "when" could be anywhere from days to decades off, which is why this is secondary to the whole gravity wave thing.
Anyway, most of the time, you're going to have clocks that are already synced, and are getting fed data from your ship's FTL computer to make any adjustments as you muck around with relativity, such that you never need to recalibrate them. So what you probably want to know is how to read them.
Internally, GTC is measures ad the number of picoseconds elapsed since an arbitrary point in time, roughly 4700 years ago, when the system was first put in place. This isn't very fun to read, so it generally breaks down as such:
- T-years: So named for Tyrol, homeworld of the Elves, because they made up the system and I guess they get to decide these things. Tyrol orbits its sun every 390 days (in Solar terms, for you humans out there).
- Sign: The Tyrolian calendar doesn't use "months" as their world has several moons with a somewhat complex orbit, but rather divides the sky into twelve constellar regions--a relic of an older astrological system. Each is equal to 32 Tyrolian solar days. There is an inter-sign period--colloquially called "The Void" for the "forbidden" 13th sign, that lasts only 6 days, at the end of each Tyrolian year.
- Day: The solar day of Tyrol is about 25.4 Earth hours long, although the Tyrolians split that into 60 subdivisions, which are (surprise surprise) about 25.4 Earth minutes long.
etc etc
You may have noticed this "galactic" system is very Tyrol-specific. That's the advantage of being top dog, I guess. Every ship is going to have its own duty cycles based on the biological makeup of the crew, and that's not likely to coincide with Tyrolian norms, even before factoring all the jet lag from landing on worlds with their own time situation. Thus, this system is mainly just used to eliminate confusion when integrating multiple different ships or worlds.
Ships often have an onboard clock based on their port of embarkation, or perhaps just an adopted standard if they aren't in port often. This is usually used more consistently than GTC.