On this day 744 years ago, on December 19, 1281 (November 7, 1281 lunar calendar), the "Time Service Calendar" was promulgated. December 19 was the day Guo Shoujing's "Time Service Calendar" was promulgated (1281). On December 19, 1281 (the 18th year of Yuan to Yuan), the Yuan Dynasty promulgated the "Time Service Calendar" by the famous astronomer Guo Shoujing, which stipulated 365.2425 days as a year. The circumference difference between the time service calendar and the earth's orbit around the sun is only 26 seconds, which is equal to the current Gregorian calendar, but appears 300 years earlier than it. "Time Service Calendar", the monthly time service calendar is 29.530593 days, with the month without middle air as the leap month. It officially abolished the ancient Shangyuan calendar year, and intercepted any year in the modern era as the calendar, breaking the ancient habit of making calendars, which is the fourth major reform in the history of our country's calendar. The "Great Unified Calendar" issued in the early Ming Dynasty is basically the "Timing Calendar". If these two editions of the calendar are regarded as one edition, it can be said that the longest edition of the calendar in our country's history has lasted 364 years. The "Time Service Calendar" is from June 1276 (the 13th year of the Yuan Dynasty) to February 1280 (the 17th year of the Yuan Dynasty). Xu Heng, Wang Xun, Guo Shoujing, Yang Gongyi, etc. established 27 test sites in a vast area of more than 6,000 miles east and west and more than 11,000 miles long in north and south, and completed the actual measurement (ie, the Four Seas Test). The Yuan Dynasty's "Yuan Dynasty Famous Ministers", "Yuan Wen Class" and other historical materials all record that Xu Heng was the editor-in-chief of the "Time Service Calendar", especially the twelfth volume of Xu Heng's posthumous "Xu Wenzheng Gong's posthumous book" has the full text of the "Time Service Calendar" - the "Time Service Calendar".