Chinese Calendar Converter

The Chinese calendar is lunisolar: months track the moon while the year tracks the sun. A month begins on the day containing the astronomical new moon as observed at 120 degrees east longitude, Beijing time at UTC+8, and runs 29 or 30 days because the mean synodic month is 29.53059 days. Twelve such months total about 354.4 days, roughly 11 short of the tropical year of 365.2422 days, so leap months are inserted seven times in each 19-year Metonic cycle. The insertion rule is precise rather than arbitrary: the year is divided by 24 solar terms, of which 12 are principal terms called zhongqi, month 11 must always contain the winter solstice, and the leap month is the first containing no zhongqi. It takes the number of the month it follows. The year label comes from the sexagenary cycle. Ten heavenly stems combine with twelve earthly branches, and because the least common multiple of 10 and 12 is 60, the pairing repeats every 60 years. The stems pair off into the five phases - Wood, Fire, Earth, Metal, Water - and the branches map onto the twelve zodiac animals, so the animal index is (year minus 4) modulo 12. Three limitations apply. This converter approximates lunar month and day from a mean lunation rather than true new moon instants, so results can differ from an official almanac by one to two days. Intercalary months are not flagged. And the zodiac boundary used is Chinese New Year, whereas bazi astrology switches at lichun, around 4 February.

Chinese calendrical reckoning is among the longest continuously documented in the world. Oracle bone inscriptions of the late Shang dynasty, roughly 1200 BC, already record days by sexagenary stem-branch pairs, and that day count has run without interruption since. The structure of the modern calendar dates to the Taichu reform promulgated under Emperor Wu of Han in 104 BC, which established the 19-year intercalation cycle and set the first month of the civil year. Later dynasties treated calendar accuracy as a matter of imperial legitimacy and reformed it repeatedly. Three reforms stand out. Zu Chongzhi's Daming calendar, completed in 462 AD, incorporated the precession of the equinoxes. The Shoushi calendar of Guo Shoujing, adopted in 1281 under the Yuan dynasty, derived a tropical year of 365.2425 days - numerically identical to the value the Gregorian reform would adopt three centuries later. In 1645 the Qing adopted the Shixian calendar, prepared with Jesuit astronomers led by Johann Adam Schall von Bell, which replaced mean solar and lunar motion with true apparent positions and so changed how month boundaries and solar terms were computed. After the Republic of China adopted the Gregorian calendar for civil use on 1 January 1912, the traditional calendar was retained for festivals and agriculture. Modern computation is fixed to UTC+8 and specified in the national standard GB/T 33661-2017, issued by the Purple Mountain Observatory.