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 (identified by name or number), and the day of the month (numbered sequentially starting from 1). Although the calendar year currently runs from 1 January to 31 December, at previous times year numbers were based on a different starting point within the calendar (see the "beginning of the year" section below). In the Julian calendar, a leap year occurred every 4 years, and the leap day was inserted by doubling 24 February. The Gregorian reform omitted a leap day in three of every 400 years and left the leap day unchanged. However, it has become customary in the modern period