On Oct. 1, 1949, Mao Zedong declared that China would follow the Gregorian calendar. For the first time the entire world agreed what the date was. Here is the first complete history of the calendar, with information about science, religion, superstition & politics of many ages. Julius Caesar attempted to impose a unified calendar, but he could not calculate exactly the length of the year. His Julian calendar gained time over the true solar year, leading to calls for reform during the Middle Ages. This caused all manner of mayhem as between 10 & 13 days were removed at a stroke, & it was 500 years before Europe was in synch again. The story of the calendar's reckoning is a tale of human will, vanity, experimentation & endeavor.