A calendar has to answer how many days there are in a year and when each solar term falls. The answer given by the Shoushi calendar was a tropical year of 365.2425 days.
In the seventeenth year of Zhiyuan (1280) Guo Shoujing, Wang Xun and others completed the Shoushi calendar, and it was promulgated the next year. The length it fixed for the tropical year is the same as that of the Gregorian calendar some three centuries later.
In the seventeenth year of Hongwu (1384) the Astronomical Bureau of the Ming took the Shoushi calendar as its base and drew up the Datong calendar, which was used until the fall of the Ming.
The Shoushi Calendar is the class in the chapter on Cosmos on the Yuan Shoushi calendar and what followed it: Guo Shoujing, Wang Xun and others completed it in the seventeenth year of Zhiyuan and it was promulgated the next year, setting the tropical year at 365.2425 days, the value of the Gregorian calendar some three centuries later; the Ming based the Datong calendar on it and used it until the dynasty fell.
