Sexagenary cycle

[1][2] It appears as a means of recording days in the first Chinese written texts, the oracle bones of the late second millennium BC Shang dynasty.

[5][6] This traditional method of numbering days and years no longer has any significant role in modern Chinese time-keeping or the official calendar.

This pattern continues until both cycles conclude simultaneously with guǐhài (癸亥), after which it begins again at jiǎzǐ.

This termination at ten and twelve's least common multiple leaves half of the combinations—such as jiǎchǒu (甲丑)—unused; this is traditionally explained by reference to pairing the stems and branches according to their yin and yang properties.

This combination of two sub-cycles to generate a larger cycle and its use to record time have parallels in other calendrical systems, notably the Akan calendar.

Eclipses recorded in the Annals demonstrate that continuity in the sexagenary day-count was unbroken from that period onwards.

The earliest discovered documents showing this usage are among the silk manuscripts recovered from Mawangdui tomb 3, sealed in 168 BC.

In one of these documents, a sexagenary grid diagram is annotated in three places to mark notable events.

[10][11] Use of the cycle to record years became widespread for administrative time-keeping during the Western Han dynasty (202 BC – 8 AD).

(However, for astrology, the year begins with the first solar term "Lìchūn" (立春), which occurs near February 4.)

[13] The Korean (환갑; 還甲 hwangap) and Japanese tradition (還暦 kanreki) of celebrating the 60th birthday (literally 'return of calendar') reflects the influence of the sexagenary cycle as a count of years.

One system follows the ordinary Chinese lunar calendar and connects the names of the months directly to the central solar term (中氣; zhōngqì).

The jiànzǐyuè ((建)子月) is the month containing the winter solstice (i.e. the 冬至 Dōngzhì) zhōngqì.

[19] In the other system (節月; jiéyuè) the "month" lasts for the period of two solar terms (two 氣策 qìcì).

The zǐyuè (子月) is the period starting with Dàxuě (大雪), i.e. the solar term before the winter solstice.

For both the stem and the branch, find the N for the row for the century, year, month, and day, then add them together.