历法

The Chinese traditional calendar is frequently called the "lunar calendar" (农历 nónglì or 阴历 yīnlì) in everyday speech, but this is an oversimplification. It is a lunisolar calendar — one that tracks both the moon's phases and the sun's annual position simultaneously, and uses an intercalary system to keep them reconciled. The Islamic calendar, by contrast, is purely lunar: its months drift freely through the solar year, which is why Ramadan can fall in any season. The Gregorian calendar is purely solar: its months bear no relationship to moon phases. The Chinese lunisolar calendar attempts to honor both celestial cycles at once, which requires a sophisticated reconciliation mechanism and produces a system of considerable astronomical elegance.

The lunar component: each month begins on the new moon (朔 shuò) and the middle of the month falls on or near the full moon (望 wàng). Months alternate between 29 and 30 days — the actual lunar month is approximately 29.53 days. Twelve lunar months total about 354 days, eleven days short of the solar year (365.25 days). Left uncorrected, the lunar months would drift steadily backward through the solar year, carrying seasonal festivals into wrong seasons within a generation.

The correction mechanism is the intercalary month: 闰月 (rùnyuè), literally "intercalary/surplus month." Approximately every 2–3 years (7 times in 19 years — the Metonic cycle, known to Chinese astronomers by at least the 5th century BCE), an extra month is inserted into the calendar. The intercalary month is a repetition of the preceding month — so a year with an intercalary month has 13 months. Which month is doubled is determined by astronomical calculation: the intercalary month is the one in which no "major solar term" (中气 zhōngqì) falls — a rule that ties the intercalation system directly to the sun's position in the ecliptic. The result is that the Chinese calendar's months drift only slowly relative to the seasons, never departing more than about a month from their canonical seasonal position.

The solar component is tracked through the 24 Solar Terms (二十四节气 èrshísì jiéqì) — fixed points in the sun's annual journey divided at 15-degree intervals. These are the agricultural spine of the calendar: they mark when to plant, when the rains arrive, when the frost begins. They are discussed in detail in the Solar Terms entry. The essential structural point here is that the Chinese calendar is not simply a moon-counting system but a dual-track astronomical instrument that uses the solar terms to anchor seasonal reality and the lunar months for everyday dating, with the intercalary mechanism keeping the two synchronized.

Alongside the month-counting calendar, the Chinese system employs a sixty-unit cycle to name years, months, days, and hours. The cycle is generated by combining two shorter sequences: the Ten Heavenly Stems (十天干 shí tiāngān) and the Twelve Earthly Branches (十二地支 shíèr dìzhī). The Stems are: 甲 jiǎ, 乙 yǐ, 丙 bǐng, 丁 dīng, 戊 wù, 己 jǐ, 庚 gēng, 辛 xīn, 壬 rén, 癸 guǐ. The Branches are: 子 zǐ, 丑 chǒu, 寅 yín, 卯 mǎo, 辰 chén, 巳 sì, 午 wǔ, 未 wèi, 申 shēn, 酉 yǒu, 戌 xū, 亥 hài.

By pairing the first Stem with the first Branch (甲子 jiǎzǐ), the second Stem with the second Branch (乙丑 yǐchǒu), and so on — with the 10-unit and 12-unit sequences cycling at different rates — the system generates 60 unique pairs before repeating (the least common multiple of 10 and 12 is 60). This sixty-unit cycle, the sexagenary cycle, has been used to name years continuously since at least the Shang dynasty (c. 1600–1046 BCE), giving China one of the longest unbroken calendrical records in world history. The year 2024 CE is 甲辰年 (jiǎchén nián) — year of the Wood Dragon.

The Earthly Branches also correspond to the twelve animals of the Chinese zodiac (十二生肖 shíèr shēngxiào): Rat (子), Ox (丑), Tiger (寅), Rabbit (卯), Dragon (辰), Snake (巳), Horse (午), Goat (未), Monkey (申), Rooster (酉), Dog (戌), Pig (亥). The zodiac is the popular face of the Branch system — widely known internationally — but the Branches themselves predate their animal associations and have a more technical role in divination, astrology, and traditional Chinese time-keeping. The twelve Branches also divide the day into two-hour periods: 子时 (zǐshí) is 11pm–1am, 午时 (wǔshí) is 11am–1pm — the origin of the noon word 中午 (zhōngwǔ), "middle of wǔ time."

The management of the calendar was a political act in imperial China, not merely an administrative one. The emperor's authority derived in part from his role as intermediary between Heaven and humanity, and the accurate regulation of time was a visible demonstration of that mandate. The Bureau of Astronomy (钦天监 Qīntiānjian, "Imperial Observatory") was a standing government department responsible for producing the official calendar — the 历书 (lìshū) — issued annually by imperial decree and distributed throughout the empire. To use an unofficial calendar was seditious: rival dynasties and usurpers issued their own calendars as assertions of legitimacy, and foreign tributary states that accepted the Chinese calendar were acknowledging the emperor's cosmic authority over time itself.

For the farming population that comprised the vast majority of the Chinese population through most of imperial history, the agricultural implications of the calendar were concrete and immediate. The solar terms (see the Solar Terms entry) specified when the seasonal qi shifts occurred; the 黄历 (huánglì, "yellow calendar" or almanac) — the popular annual publication derived from the official calendar — gave day-by-day auspiciousness ratings for agricultural, commercial, construction, marriage, and travel activities. The 黄历 was the most widely distributed text in China for centuries, found in virtually every household. It assigned each day a set of favorable and unfavorable activities based on the interaction of the day's Stem-Branch designation with the lunar month and solar term.

The relationship between the calendar and traditional Chinese medicine is not metaphorical — it is structural. The Huangdi Neijing teaches that the body's qi patterns change with the seasons, mirroring the macro-cosmic qi shifts tracked by the solar terms. The liver is associated with spring and the rising wood energy of the east; the heart with summer and fire; the spleen with late summer and earth; the lungs with autumn and metal; the kidneys with winter and water. A practitioner treating the same pattern in summer and in winter will use different points, different herbs, and different dietary recommendations — because the same treatment does not have the same effect in different seasonal contexts.

The most systematic expression of this calendar-medicine integration is the system of 子午流注 (zǐwǔ liúzhù, "midnight-noon ebb-and-flow") — an advanced acupuncture scheduling system in which specific meridians and acupoints are considered most accessible at specific two-hour periods within the day (corresponding to the twelve Branches), specific days of the month, and specific months of the year. A practitioner using this system selects acupoints not only based on the patient's pattern but based on when in the temporal cycle the treatment occurs. The theory is that qi is distributed unevenly through the channel network across time, and that treating a channel when its qi is at peak or nadir produces stronger effects. This system requires considerable astronomical and calendrical literacy to apply and represents one of the more technically demanding intersections of cosmological thought and clinical practice in Chinese medicine.

The Republic of China formally adopted the Gregorian calendar (公历 gōnglì or 阳历 yánglì) for civil purposes in 1912, and the People's Republic continued this practice. In contemporary China, the Gregorian calendar governs business dates, government documents, and the official workweek. Yet the traditional lunisolar calendar (农历 nónglì, "agricultural calendar," or 阴历 yīnlì, "yin calendar") remains the structuring framework for the major festivals, auspicious date selection, and much of everyday cultural life. Spring Festival (春节 Chūnjié) falls on the first day of the first lunar month — its Gregorian date shifts each year. Mid-Autumn Festival falls on the fifteenth of the eighth lunar month. The Lantern Festival on the fifteenth of the first lunar month. Dragon Boat Festival on the fifth of the fifth. None of these have fixed Gregorian dates because none are defined by the Gregorian calendar.

Auspicious date selection (择日 zérì) — choosing the right day for weddings, business openings, moving house, funerals, and major decisions — remains a significant practice in mainland China, Taiwan, Hong Kong, and the Chinese diaspora, and it operates entirely within the traditional calendrical framework. The digital 黄历 (almanac) is one of the most-accessed features on major Chinese calendar apps: people check the day's rating the same way they might check the weather. This is not uniformly believed as literal cosmological truth — many users treat it as cultural habit, a soft acknowledgment of tradition — but its continued use indexes the degree to which the traditional calendar structures temporal experience for hundreds of millions of people alongside the Gregorian system they use professionally.