Chinese calendar by mountain peoples. After Qin Shi Huang unified China under the Qin dynasty in 221 BC, the Qin calendar () was introduced. It followed most of the rules governing the Zhuanxu calendar, but the month order was that of the Xia calendar; the year began with month 10 and ended with month 9, analogous to a Gregorian calendar beginning in October and ending in September. The intercalary month, known as the second "Jiǔyuè" (), was placed at the end of the year. The Qin calendar was used into the Han dynasty. Emperor Wu of Han introduced reforms halfway through his
Gregorian calendar It also specifies its epact relative to 1 January, in contrast with the Julian calendar, which specified it relative to 22 March. The old date was derived from the Greek system: the earlier "Supputatio Romana" specified it relative to 1 January. During the period between 1582, when the first countries adopted the Gregorian calendar, and 1923, when the last European country adopted it, it was often necessary to indicate the date of some event in both the Julian calendar and in the Gregorian calendar, for example, "10/21 February 1750/51", where the dual year accounts for some countries already beginning their