The Chinese calendar

Fact sheet

Scientific classification

Calendars are usually classified on two axes: by the unit they rest on (solar, lunar, lunisolar), and by the method of calculation (observational, relying on sighting; arithmetical, with fixed rules and averages; or astronomical, by the true positions of the bodies). The Chinese calendar is:

• Lunisolar: its months are true lunar months, and its year stays with the seasons because a leap month is added in about seven years out of every nineteen.
• Astronomical today: since 1645 the month is computed from the true new moon and the terms from the true position of the sun, which is why Reingold and Dershowitz place it among the astronomical rather than arithmetical calendars in Calendrical Calculations. It passed through three stages: arithmetical with mean values until 619, then mixed (true new moon, mean terms) until 1644, then fully astronomical.
• Not observational: it does not rely on sighting the crescent; from about the 7th century BCE the first of the month has been computed instead of observed.
• Its standing in scholarship: one of the longest-lived and best-documented calendars, studied in the history of Chinese astronomy as a continuous record of the development of astronomical measurement over more than two thousand years; its count of days by the sexagenary cycle is considered unbroken since at least the 8th century BCE, which makes it a tool for dating ancient events and eclipses.

Current rules (standard GB/T 33661‑2017)

1. The day begins at midnight Beijing time (the 120° E meridian, UTC+8).
2. The month begins on the day on which the new moon falls — the conjunction of the moon and the sun in ecliptic longitude — and the whole day of the new moon belongs to the month whatever its hour. So a month has 29 or 30 days, and two long or two short months may follow one another.
3. The eleventh month is always the month in which the winter solstice (solar longitude 270°) falls. The span from one eleventh month to the next is called a “suì” 岁.
4. A leap suì: if a suì has 13 months (i.e. 13 new moons fall between two winter solstices), the first month in it that contains no “principal term” 中气 — the sun reaching a multiple of 30° in longitude — is the leap month; it takes the number of the month before it, preceded by the word “leap” 闰: leap sixth month 闰六月 follows the sixth.
5. The year begins with the first month 正月, usually the second month after the month of the winter solstice, and ends with the twelfth month. It has 12 or 13 months depending on whether it contains a leap month.

Why the “suì” and not the year? Because the leap month is determined between two winter solstices, while the year begins about two months after the solstice. So a year may be a leap year while the suì that begins in it is common, or the reverse. This is what tripped up older computations in the “2033 problem” (see the modern era).

Computed examples (New Year and leap months)

The twenty-four solar terms

The ecliptic is divided into 24 sections of 15°, and the sun’s reaching each boundary is a “term” 节气 whose name describes the weather or farming on the North China Plain. The terms at multiples of 30° are the “principal terms” 中气 that determine the leap month (shaded in the table); the others are the “sectional terms” 节. Dates are approximate and shift by a day between years. “The Twenty-Four Solar Terms” were inscribed on UNESCO’s list of Intangible Cultural Heritage in 2016.

In Chinese reckoning the “start of spring” 立春 falls around 4 February, so the Chinese seasons begin about 45 days before the astronomical ones: the Chinese spring is centred on the spring equinox rather than beginning with it.

Months, years and the sexagenary cycle

The months

The months are numbered, not named: the first 正月 (zhēngyuè), the second 二月 … to the twelfth. The last two have common names: the eleventh, “winter month” 冬月, and the twelfth, “làyuè” 腊月. Days are written with special numerals in the first ten: 初一 (the first) … 初十, then 十一 … 二十, then 廿一 … and 三十.

The sexagenary cycle (stems and branches)

Years and days (and historically months and hours) are named by pairs of ten heavenly stems and twelve earthly branches, which advance together so that the same pair recurs every 60. The cycle begins with 甲子; 1984 was such a year, so 2044 begins a new cycle.

So 2026 is 丙午: the stem 丙 (fire) and the branch 午 (horse), the “year of the fire horse.” The zodiac animal changes at the Chinese New Year, not on 1 January.

Counting the years

• Reign eras 年号: from 140 BCE until the fall of the Qing, years were counted by the names of emperors’ eras.
• The Republic era 民国: begins in 1912 and is still used in Taiwan (2026 = year 115).
• The Yellow Emperor era: a continuous nationalist count promoted by some intellectuals in the late Qing; its number depends on the chosen epoch (2026 = 4723 or 4724).
• Today: the Gregorian year is given with its sexagenary name, which is what this site uses.

History: from the oracle bones to 1645

A large number of official calendars were issued in China over its history — between sixty and a hundred, depending on what counts as a separate issue — because every ruling dynasty saw in an accurate calendar a sign of the “Mandate of Heaven.” These are the main milestones:

1. c. 1250 BCE

   The Shang: oracle-bone inscriptions

   The oracle-bone inscriptions show a lunisolar year of 12 months to which a thirteenth month was sometimes added at the end, and a count of days by the sexagenary cycle.

2. 7th–6th c. BCE

   From sighting to calculation

   Under the Zhou, in the Spring and Autumn period, the first of the month came to be computed from the new moon instead of sighting the crescent (about 621 BCE), and the rule of seven leap months in every 19 years became established (after about 589 BCE).

3. 5th–3rd c. BCE

   The six ancient calendars

   In the Warring States period there were several calendars (known as the Huangdi, Xia, Yin, Zhou, Lu and Zhuanxu), all of the “quarter remainder” type 四分历: a year of 365¼ days and a 19-year cycle of 235 months. They differed in the month that began the year.

4. 221 BCE

   The Qin: one calendar for the empire

   The Qin unified the calendar on the Zhuanxu rules: the year began with the tenth month, and the leap month was placed at its end as a “later ninth month” 后九月.

5. 104 BCE

   The Taichu calendar 太初历

   The reform of Emperor Wu of Han laid the foundations of the calendar that last to the present: the year begins with the month two months after the month of the winter solstice (the “Xia” rule 建寅), the twenty-four terms enter the calendar, and the leap month is the month without a principal term. Its mean month is 29 and 43/81 days (≈ 29.5309 days), so it was known as the “eighty-one-parts calendar” 八十一分历; the months written in the calendar have whole 29 or 30 days, alternating so that their average stays at this value.

6. 85 CE

   Back to the “quarter remainder”

   The Eastern Han adopted the Sifen calendar 四分历, more accurate in the length of the year, keeping the fixed nineteen-year cycle.

7. 223 CE

   The Qianxiang calendar 乾象历

   Composed by Liu Hong at the end of the Han and adopted by the kingdom of Wu, it brought the moon’s irregular motion (the anomalistic month) into the calculation, although the month used remained the mean month.

8. 462 / 510 CE

   The Daming calendar 大明历

   Composed by Zu Chongzhi in 462 and adopted in 510, it introduced the precession of the equinoxes, distinguishing the tropical year from the sidereal, and proposed a 391-year cycle with 144 leap months instead of the nineteen-year cycle.

9. 619 CE

   The Tang: the true new moon

   The Wuyin calendar 戊寅元历 was the first to begin the month with the true rather than the mean new moon, so the month became 29 or 30 days according to the moon’s actual motion.

10. 665 CE

    The Linde calendar 麟德历

    Composed by Li Chunfeng, it fixed the true new moon in the calendar for good and abandoned the fixed nineteen-year cycle, so that the leap month followed the terms rather than a table.

11. 729 CE

    The Dayan calendar 大衍历

    Composed by the monk Yixing after a wide measurement campaign, it used the sun’s true motion in computing eclipses; the terms written in the calendar, however, remained mean (equal parts of the year) until 1645.

12. 1281 CE

    The Yuan: the Shoushi calendar 授时历

    Composed by Guo Shoujing and his team with new instruments and observatories, at a time when the court had a bureau of Muslim astronomers; its year is 365.2425 days — the same value the Gregorian reform adopted a century later. It is the longest-lived of the traditional calendars and the last made by purely Chinese methods.

13. 1368–1644

    The Ming: the Datong calendar 大统历

    It is the Shoushi calendar with a slight change of epoch, used together with the translated “Muslim calendar” 回回历 for eclipses. As its errors in predicting eclipses accumulated, Xu Guangqi was charged in 1629 with preparing a new calendar on European astronomy with the help of the Jesuits, producing the “Chongzhen calendar books” 崇祯历书; but the Ming fell (1644) before it was adopted.

14. 1645 CE

    The Qing: the Shixian calendar 时宪历

    The German Jesuit Adam Schall von Bell presented it to the Shunzhi Emperor on the basis of the Chongzhen books, and it was adopted. Its key feature is the true terms 定气 (the sun actually reaching each 15°) instead of equal parts, so the calendar became astronomical for both moon and sun, and a single month could now contain two principal terms. It also fixed the month of the winter solstice as the eleventh.

From 1645 to the present

1. 1664–1669

   The “Calendar Case”

   Yang Guangxian accused the Jesuits and the new calendar was temporarily abolished; then Ferdinand Verbiest proved the Western computation correct in a test before the court, and it was readopted in 1669 with Verbiest heading the astronomical bureau.

2. 1742

   The Guimao revision

   The solar and lunar theories of the calendar were updated with more accurate theories of the motion of the sun and the moon and newer European observations; the rules stayed the same while accuracy rose, and this lasted to the end of the Qing.

3. 1912

   The Republic: the Gregorian becomes official

   On 2 January 1912 Sun Yat-sen announced the adoption of the Gregorian calendar and the Republic era, while the old calendar remained for festivals and daily life. The Nationalist government tried to ban it between 1929 and 1934 without success, and since 1947 it has been called the “nónglì” (agricultural calendar).

4. 1929

   From the Beijing meridian to 120° E

   Computation had used Beijing local time (116°25′ E); then the 120° meridian (UTC+8) was adopted after time zones were set. The difference is about 14 minutes, so a new moon near midnight can shift the start of a month by a day: the new moon of September 1978 fell at 23:53 on 2 September on the old Beijing meridian and at 00:07 on 3 September in official time, so the eighth month began on 3 September.

5. since 1949

   The People’s Republic and the Purple Mountain Observatory

   The People’s Republic of China adopted the Gregorian calendar, and the Purple Mountain Observatory in Nanjing issues the lunar calendar from modern astronomical computations. Popular almanacs in Hong Kong and Taiwan kept following the computations of the 1908 imperial calendar until 1978, and then agreed with the modern computation.

6. 2016

   UNESCO

   “The Twenty-Four Solar Terms” were inscribed on the Representative List of the Intangible Cultural Heritage of Humanity.

7. 2017

   National standard GB/T 33661‑2017

   On 12 May 2017 the Standardization Administration of China issued the standard “Calculation and promulgation of the Chinese calendar” 农历的编算和颁行, codifying the five rules above officially for the first time.

8. 2033

   The “2033 problem”

   In the suì of 2033–2034 there is a month without a principal term that is not the leap month, and another month with two principal terms — a situation that can arise only with true terms after 1645. Almanacs in circulation until the early 1990s placed the leap month of 2033 after the seventh month; by the suì and winter-solstice rule (the standard’s rule) it is correctly the leap eleventh month.

In East Asia

The Chinese calendar spread to Korea, Vietnam, Japan and the Ryukyu Islands, each of which computed it on its own meridian, so dates sometimes differ by a day or a month:

• Vietnam: on 8 August 1967 North Vietnam adopted a lunar calendar on UTC+7, so the New Year (Tết) of 1968 fell on 29 January in the north and 30 January in the south, which stayed on China’s time — which confused the timing of the “Tet Offensive” that year.
• Korea: computed the calendar on the Seoul meridian from the 15th century, officially adopted the Gregorian calendar in 1896, and still computes the lunar calendar — including the Korean Lunar New Year, “Seollal” 설날 — on its own time (UTC+9), so it sometimes differs from the Chinese by a day.
• Japan: its calendar was derived from the Chinese on a Japanese meridian, and it abolished its official use in 1873 in favour of the Gregorian.

The Chinese calendar on this site

• It is computed astronomically by the rules of standard GB/T 33661‑2017 from NASA’s DE441 data: the new moons and the true longitude of the sun, on UTC+8 since 1929 and Beijing local time before it.
• It matches the Hong Kong Observatory tables day by day from 1901 to 2100; in three cases where the new moon falls minutes from midnight (1906, 2057 and 2097) the official month start in those tables is used.
• From 1645 to 1911 the modern computation agrees with the Qing calendars in most months and differs on a few days, because those were computed with older theories. Before 1645 the computation extends the modern rules, so it does not match the calendars of the old dynasties described above.
• The year is written as the Gregorian year in which it begins, with its sexagenary name and zodiac animal.

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