The Galactic Standard Date (GSD) is a wide-spread means of time-keeping. While “Standard Date” is the official term, it’s considered “Galactic Standard” by the Fed and its Colonial Authorities and “Federal Standard” by non-Fed states.
The 360-day Standard Year (SY) is based on no specific orbital period or rotational period. Rather, it is officially based on an average of the orbital periods of several human-inhabited planets, with the 25-hour Standard Day based on a rounded average of broadly sampled human biorhythms. Unofficially, there’s 360 ° to a circle and the math from there is just… easier.
The epoch of the Standard Calendar was retroactively set as the point of first contact between subspecies of the Human Diaspora.
While the exact math would be a bit rough, the Standard Date can be converted to the Gregorian calendar by adding 1399 years.
GSD came about by effectively “rounding down” from the nicely obvious 1492. A period of approximately 600 years seemed like more than enough time for the human diaspora to get up, have a nice big war, recover, get out, flatten out their tech distribution curve a bit, and then start growing again.
GSD is also an obtuse, confusing wank and as such its use has been thoroughly deprecated in all but the most applicable situations.