The galactic standard calendar was a common standard for measuring time across the galaxy, regardless of the local differences. It was based on the orbital motion of the planet Coruscant, and its primary moon Centax-1.
Its fundamental unit, the standard day, was based on one complete revolution of Coruscant. The planet took 368 standard days to make one complete orbit of its star, Coruscant Prime, a time period that became the standard year.
The year was divided into standard months and standard weeks. The length of a month, 35 days, was determined by the syzygy of Coruscant's primary moon Centax-1—the time it took to pass from one new (or full) moon to the next. The month was divided into seven weeks, each containing five days named after celestial bodies or historical inhabitants of Coruscant: Primeday, Centaxday, Taungsday, Zhellday, and Benduday.
There were 10 standard months in a standard year, which only added up to 350. Therefore, the "missing" 18 days were made up of three festival of fete weeks, plus three regular annual holidays. New Year Fete Week fell in the first five days of the new year, before the first month, while the Festival of Life fete occurred between the sixth and seventh, and the Festival of Stars between the ninth and tenth. The three holidays varied depending on the local culture. For example, Boonta Eve and Harvest Day were only observed on Tatooine.
The galactic standard calendar was introduced under the Old Republic. Although some cultures kept measuring time in units other than those used by the standard, the system came to be understood in most places.
- Star Wars: Build the Millennium Falcon 61 (Guide to the Galaxy: Galactic Time and Dates) (First identified as galactic standard calendar)