Modern Chinese use two calendar systems. They use the Gregorian (Western / Yang) calendar to conduct official business and to manage everyday life. At the same time, they use the Chinese (Lunar / Yin) calendar to determine the numerous celebration dates; to find out the dates of agriculture; and to calculate human fortunes with techniques like Zi Wei Dou Shu, Bazi, Chinese Gender Chart and Tik Pan San Sou.
The Chinese calendar is based on the period of time that the moon needs to travel a full cycle (new, half, and full and back). Therefore, it is also called the lunar calendar. Like a Gregorian year, a normal Chinese year consists of twelve lunar months. However, as the moon only takes approximately 29 ½ days to complete a cycle, it makes the lunar year eleven days shorter than the Western calendar year. Every nineteen years, the western and Chinese calendars sync up with the aid of seven lunar leap months. In other words, there are 7 lunar leap years (13 lunar months each) in a nineteen-year cycle.
The use of the Chinese calendar dates back to 2600 B.C. when Emperor Huang Ti introduced the twelve periods of the zodiac to the Country. Later, Chinese astronomy further divides the lunar year into twenty-four segments (Table 1). Each segment is based on the longitude of the sun on the ecliptic or solar terms, and therefore, the Chinese calendar also belongs to one of the lunisolar calendars. That is why the Winter Commences is always on Dec 21st or Dec 22nd, as it is the shortest day of the year; and the Summer Commences is always on Jun 21st or Jun 22nd, as it is the longest day of the year. The lunar calendar functions as a weather calendar for farmers, who use it to guide their crop planning in China.
Table 1: Twenty-Four Segments of the Chinese Calendar