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.