The Chinese calendar is a lunisolar calendar formed by combining a purely lunar calendar with a solar calendar. Among Chinese, the calendar is not used for most day to day activities, but is used for the dating of holidays such as Chinese New Year (Spring Festival) and the Mid-Autumn Festival and for divination. The primary use in day to day activities is for determining the phase of the moon, which is important for farmers and is possible because each day in the calendar corresponds to a particular phase of the month.

In China, the native calendar is the "farmer's calendar", as opposed to the "civil calendar", or "Western calendar".

History
The legendary beginning of the Chinese calendar developed during the first millennium BC. The legend states that the first Chinese calendar was invented by the first legendary emperor, Huangdi or the Yellow Emperor, whose reign was assigned to 2698-2599 BC. The fourth legendary emperor, Emperor Yao, added the intercalary month. The 60-year stem-branch cycle was first assigned to years during the first century BC. Giving Huangdi some maturity, the first year of the first cycle was assigned to 2637 BC according to Herbert A. Giles, A Chinese-English Dictionary (1912), and all other authors of the late Qing dynasty. Thus since 1984 the current cycle has been 78. However, some modern authors assign the first year of the first cycle to 2697 BC while Huangdi was still immature, saying we are now in cycle 79. These two epochs give rise to two continuous counts of years popular among Western chronologists¡ªcausing the Chinese years 4641 or 4701 to begin in early 2004. However, overseas Chinese communities (excluding those in southeast Asia) like San Francisco's Chinatown follow the early Chinese Republicans of the late Qing dynasty, including Sun Yat-sen, in using a 2698 BC epoch, causing 4702 to begin in early 2004.

The earliest archaeological evidence of the Chinese calendar appears on oracle bones of the late second millennium BC Shang dynasty. show a 12-month lunisolar year having an occasional thirteenth month, and even a fourteenth month. Because Chinese dates are on firm ground beginning in 841 BC, the calendar of the early Zhou dynasty known to have used arbitrary intercalations. The first month of its year was near the winter solstice and its intercalary month was after the twelfth month. The s¨¬f¨¥n (quarter remainder) calendar, which began about 484 BC, was the first calculated Chinese calendar, so named because it used a solar year of 365? days, along with a 19-year = 235-month Rule Cycle, known in the West as the Metonic cycle. The winter solstice was in its first month and its intercalary month was inserted after the twelfth month. Beginning in 256 BC with the Qin kingdom, which would later become the Qin dynasty, the intercalary month was an extra ninth month at the end of a year that began with the tenth month, now placing the winter solstice in the eleventh month.

The great Emperor Wu of the Western Han dynasty introduced the basic rules that have governed the Chinese calendar ever since. His T¨¤ich¨± (Grand Inception) calendar of 104 BC had a year with the winter solstice in the eleventh month and designated as intercalary any calendar month (a month of 29 or 30 whole days) during which the sun remained within the same sign of the zobiac throughout. Because the sun's mean motion was used to calculate the ji¨¦q¨¬ until 1645, this intercalary month was equally likely to occur after any month of the year. However, the conjunction of the sun and moon (the astronomical new moon) used the mean motions of both the sun and moon only until 619, the second year of the Tang dynasty, when both began to use true motions modeled using two offset opposing parabolas (with small linear and cubic components). Unfortunately, the parabolas did not meet smoothly at the mean motion, but met with a discontinuity or jump.

With the introduction of Western astronomy into China via the Jesuits, the motions of both the sun and moon began to use sinusoids in the 1645 Sh¨ªxi¨¤n (Constant Conformity) calendar of the Qing dynasty. The true motion of the sun was now used to calculate the ji¨¦q¨¬, which caused the intercalary month to often occur after the second through the ninth months, but rarely after the tenth through first months. A few autumn-winter periods have one or two calendar months where the sun enters two signs of the zodiac, interspersed with two or three calendar months where the sun stays within one sign.

The Gregorian calendar was adopted by the nascent Republic of China effective January 1, 1912 for official business, but the general populace continued to use the traditional calendar of the Qing Dynasty. The status of the Gregorian calendar between about 1916 and 1921 while China was controlled by several competing warlords is unknown. From about 1921 until 1928 warlords continued to control northern China, but the Kuomintang controlled sourthern China and probably used the Gregorian calendar. After the Kuomintang declared a reconstituted Republic of China October 10, 1928, it decreed that effective January 1, 1929, everyone must use the Gregorian calendar. It also decreed that effective January 1, 1929, all of China must use the coastal time zone that had been used by all European treaty ports along the Chinese coast since the late nineteenth century. This changed the beginning of each calendar day, for both the traditional and Gregorian calendars, by +14.3 minutes from Beijing midnight to midnight at the longitude 120¡ã east of Greenwich. The Kuomintang may have begun to number the years of their republic in 1929, regarding 1912 as year 1. When the Communites gained control of mainland China october 1, 1949, they simply continued using the Gregorian calendar, but now numbered the years in the Western manner, beginning with 1949. On both mainland China and Taiwan, the months of the Gregorian calendar are numbered 1-12 just like the months of the traditional calendar.

Rules
The following rules have been valid since 104 BC, although some of the details were unnecessary before 1645. Note that these rules do not specify the detailed calculations, permitting either mean or true motions of the Sun and Moon to be used, depending on the historical period.

1. The months are lunar months, such that the first day of each month beginning at midnight is the day of the astronomical new moon.
2. Each year has 12 regular months, which are numbered in sequence (1 to 12). Every second or third year has an intercalary month, which may come after any regular month. It has the same number as the preceding regular month, but is designated intercalary.
3. Every other ji¨¦q¨¬ of the Chinese solar year is equivalent to an entry of the sun into a sign of the tropical zodiac (a principle term or cusp).
4. The sun always passes the winter solstice (enters Capricorn) during month 11.
5. If there are 12 months between two successive occurrences of month 11, at least one of these 12 months must be a month during which the sun remains within the same zodiac sign throughout (no principle term or cusp occurs within it). If only one such month occurs, it is designated intercalary, but if two such months occur, only the first is designated intercalary.
6. The times of the astronomical new moons and the sun entering a zodiac sign are determined in the Chinese Time Zone by the Purple Mountain Observatory outside Nanjing using modern astronomical equations.

The Zodiac Sign which the sun enters during the month and the ecliptic longitude of that entry point usually determine the number of a regular month. Month 1, zh¨¨ngyu¨¨, literally means principal month. All other months are literally numbered, second month, third month, etc.

Some believe this correspondence to be always true, but there are exceptions, which, for example, prevent Chinese New Year from always being the second new moon after the winter solstice. Such an exception occurred in 1984-85, after the sun had entered both Capricorn at 270¡ã and Aquarius at 300¡ã in month 11, and then entered Pisces at 330¡ã during the next month, which should have caused it to be month 1. The sun did not enter any sign during the second month after month 11. In order to keep the winter solstice in month 11, the month which should have been month 1 became month 12, and the month thereafter became month 1, causing Chinese New Year to occur on 20 February 1985 after the sun had already passed into Pisces at 330¡ã during the previous month, rather than during the month beginning on that day. The problem here is that there is a month in which the sun enters two signs of the zodiac. Here such a month will be referred to as a dual-entry month. If a given month is a dual-entry month or has a dual-entry month nearby (within three months), the above correspondence may fail, otherwise it holds.

On those occasions when a dual-entry month does occur, it always occurs somewhere between two months that do not have any entry (non-entry months). It usually occurs alone and either includes the winter solstice or is nearby, thus placing the winter solstice in month 11 (rule 4) chooses which of the two non-entry months becomes the intercalary month. In 1984-85, the month immediately before the dual-entry month 11 was a non-entry month which was designated as an intercalary month 10. All months from the dual-entry month to the non-entry month that is not to be intercalary are sequentially numbered with the nearby regular months (rule 2). The last phrase of rule 5, choosing the first of two non-entry months between months 11, is only needed once, in 2033-34, when two dual-entry months will be interspersed among three non-entry months, two of which will be on one side of month 11.

Exceptions such as these are rare. Fully 96.6% of all months contain only one entry into a zodiacal sign (have one principle term or cusp), all obeying the numbering rules of the ji¨¦q¨¬ table, and 3.0% of all months are intercalary months (always non-entry months between principle terms or cusps). Only 0.4% of all months either are dual-entry months (have two principle terms or cusps) or are neighboring months that are renumbered.

The Chinese lunar calendar and the Gregorian Calendar often sync up every 19 years (Metonic cycle). Most Chinese people notice that their Chinese and Western birthdays often fall on the same day on their 19th, 38th birthday etc. However, a 19-year cycle with an certain set of intercalary months is only an approximation, so an almost identical pattern of intercalary months in subsequent cycles will eventually change after some multiple of 19 years to a quite different 19-year cycle.

The Chinese zodiac (see Nomenclature and Twelve Animals sections) is only used in naming years¡ªit is not used in the actual calculation of the calendar. In fact, the Chinese have a very different constellation system.

Nomenclature
The years are named by a cycle of 10 Heavenly Stems and a cycle of 12 Earthly Branches. Each year is named by a pair of one stem and one branch called a Stem-Branch. The Heavenly Stems are associated with Yin Yang and the Five Elements. Recent 10-year periods began in 1984, 1994, 2004, etc. Earthly Branches are associated with the Twelve Animals. Recent 12-year periods began in 1984, 1996, etc.

The 60-year cycle formed by combining the two cycles is known as a ji¨£z¨«.It is not 120 because half of the combinations are unused. Ji¨£z¨« is named after the first year in the 60-year cycle which is called ji¨£z¨«. Some figures of speech use "ji¨£z¨«" to mean "a full lifespan"¡ªone who has lived more than a ji¨£z¨« is obviously blessed. (Compare the Biblical "three-score years and ten.")

This 60-year cycle is insufficient for historical references. During feudal China, the Nian Hao (Era name of an emperor) was placed in front of the year name for distinction.

The months, days, and hours can also be denoted using Heavenly Stems and Earthly Branches, though they are commonly addressed using Chinese numerals instead. Together, four Stem-Branch pairs form the Eight Characters used in Chinese astrology.

There is a distinction between a solar year and a lunar year in the Chinese calendar because the calendar is lunisolar. A lunar year is from one Chinese new year to the next. A solar year is either the period between one "start of spring" and the next or the period between two winter solstices (see Ji¨¦q¨¬ section). A lunar year is exclusively used for dates, whereas a solar year, especially that between winter solstices, is used to number the months.

Twelve Animals
The Twelve Animals representing the twelve Earth Branches are, in order, the rat, ox, tiger, rabbit, dragon, snake, horse, sheep (or goat), monkey, rooster, dog, and pig.

A legend explains the sequence in which the animals were assigned. Supposedly, the twelve animals fought over the precedence of the animals in the cycle of years in the calendar, so the Chinese gods held a contest to determine the order. All the animals lined up on the bank of a river and were given the task of getting to the opposite shore. Their order in the calendar would be set by the order in which the animals managed to reach the other side. The cat wondered how he would get across if he was afraid of water. At the same time, the ox wondered how he would cross with his poor eyesight. The calculating rat suggested that he and the cat jump onto the ox's back and guide him across. The ox was steady and hard-working so that he did not notice a commotion on his back. In the meanwhile, the rat snuck up behind the unsuspecting cat and shoved him into the water. Just as the ox came ashore, the rat jumped off and finished the race first. The lazy pig came to the far shore last. And so the rat got the first year named after him, the ox got the second year, and the pig ended up as the last. The cat finished too late to win any place in the calendar, and became the sworn enemy of the rat.

Ji¨¦q¨¬
Chinese months follow the phases of the moon. The part of the calendar that follows the movement of the sun is called ji¨¦q¨¬.Ji¨¦q¨¬ can be translated as "Solar Terms" (lit. Nodes of Weather). There are twenty four ji¨¦q¨¬. Each is the instant when the sun reaches one of twenty four equally spaced points along the ecliptic, including the solstices and equinoxes, thus they are at fifteen-degree intervals. In the table below, these points are given in the Western form, where zero degrees is the ecliptic longitude of the vernal equinox. Because their calculation is solar-based, these ji¨¦q¨¬ roughly fall on the same dates in solar calendars such as the gregorian calendar, but do not form any obvious pattern in the Chinese calendar. The dates below are approximate and may vary slightly from year to year due to the intercalary rules of the Gregorian calendar. Before the Gregorian calendar was introduced into China, ji¨¦q¨¬ were published each year in farmers' almanacs. Farmers relied on these ji¨¦q¨¬ to plan their planting and harvest seasons. Chinese New Year is usually the new moon day closest to l¨¬ch¨±n. Each calendar month under the heading "M" contains the designated ji¨¦q¨¬ called a principle term, which is an entry into a sign of the zodiac, also known as a cusp. Here term has the archaic meaning of a limit, not a duration. Chinese seasons are centered on the solstices and equinoxes, whereas in the West, they begin at the solstices and equinoxes. Spring Festival (Chinese New Year) marks the beginning of spring, even though it usually occurs in February.