I have some ideas for how to synchronize a galactic time system given the constraints of modern physics. However, my understanding of relativity is pretty limited so I was hoping for feedback or corrections if what I'm suggesting doesn't work, or if there are better ways of doing it. Thanks!
In my setting, there is an established galactic community, but it is constrained by the limits of currently-understood physics. This means that travel between solar systems takes dozens of years for close ones, and requires massive nuclear-propelled starships to send even a small payload. The result of this is that almost all galactic trade, interaction, and commerce is digital, with comm-buoys set up orbiting most stars to relay messages between empires. Transmitting a message across the galaxy will still take 100,000 years, but this is far faster/cheaper than conventional travel.
Under this system, most everyday timekeeping is kept local (planet rotations, etc), due to relativity making time pass at different speeds in different places. But, there is a desire to have some sort of standard that people can share & convert from. So someone can say "the war lasted 10 [GALACTIC TIME UNITS]", and we can convert it to earth years ourselves.
Here is the system I came up with:
For most everyday people, the time-units they deal with are entirely local to their setting, so people on Earth still use seconds, days, years, etc.
People on ships traveling at relativistic speeds will use the time system of their birth world, but will localize it and count up using onboard clocks from the moment their ship was first commissioned (with that being their Year 0). Ships will also manage a local day-night cycle like the planet their species first evolved on. This lets people get on with their everyday lives without having to think about it.
For scientific measurements, there is a galactic unit of time based off the half-life of a known atom. This should be the same anywhere (right?), albeit relative to the speed at which time is passing in that place. So a scientist can describe an experiment in these units, but it's not useful for knowing how long ago for you the Glubglub War ended since time on Glubglug passes differently.
The Hard Part:
The tricky part comes in when syncing up a common understanding of the passage of time. What I want is for the galaxy to have determined one singular "location" where time passes canonically, and everyone else converts to/from that for their local needs. This way you can say "this happened at 147 Galactic Standard Time" and everyone immediately knows that that means. The problem is, what to synchronize off of, and is said synchronization possible? There is a good answer here, which is similar to the above, but is skirts around that issue.
One idea I had was to have everyone synchronize themselves off a commonly observable stellar phenomenon. Galactic rotation seems too slow and imprecise, but if we can theoretically measure it with any kind of precision it would be perfect. (It's probably OK for local clocks to "drift" a bit, as long as they can eventually resynchronize so the error doesn't compound).
Is there some star or black hole in our galaxy that sends out a burst of light or radiation at set times which everyone can observe and measure? If so then it would be perfect as everyone can just check their clocks against said star.
Another idea is to have one of the founding members of this Galactic Community just set their home planet as Galactic Standard Time, and make it's gravity/relativistic time speed known to everyone. This means that once someone knows how fast time is passing at said planet, they can convert it to their local time. The problem here is everyone needs to maintain 2 clocks, one for their local time, and one that tries to track the time on a planet where time passes at a different speed. The question then becomes, do you always know how fast time is passing for you, to allow for said conversion?
Finally, is there some other system I'm not thinking of which would do the trick?
Thanks, I appreciate the help/input!