What is the Ancient Chinese time recording system?

The Chinese sexagenary cycle (Chinese: 六十甲子), is a cycle of sixty terms used for recording hours, days, months and years. It is also called the Stems-and-Branches System (Chinese: 干支). The earliest written record of this system dates back to 1250BC of the Shang Dynasty, as inscriptions of divination on an oracle bone. The mathematical principles of the compound 60-cycle and its timekeeping function have strongly influenced other neighbouring cultures, notably in Japan, Korea and Vietnam.

Records of divination on oracle bones around 1250BC in Shang Dynasty

Each character in the sexagenary cycle (‘cycle of 60’) consists of a pillar, which in turn consists of a pair of Chinese characters. The first character is called Heavenly Stems (Chinese: 天干tian gan), which is one of the ten terms forming a ‘cycle of 10’. The second is called Earthly Branches (地支 di zhi), which is one of the 12 terms forming ‘a cycle of 12’.

The cycle of 10 Heavenly Stems: 甲 Jia、乙 Yi、丙 Bing、丁 Ding、戊 Wu、己 Ji、庚 Geng、辛 Xin、壬 Ren、癸 Gui The cycle of 12 Earthly Branches：子 Zi、丑 Chou、寅 Yin、卯 Mao、辰 Chen、巳 Is、午 Wu、未 Wei、申 Shen、酉 You、戌 Xu、亥 Hai

The 60-cycle envisaged as a pair of toothed wheels representing the 10-cycle and the12-cycle. For example, the first term in the cycle of 60 is 甲子 (refer to the table below), a combination of the 1st character in the 10-cycle (甲) and the 1st character in the 12-cycle (子). Then乙丑、丙寅、丁卯…etc. After 癸酉is 甲戌, which is the combination of 1st character in the 10-cycle and the 11th character in the 12-cycle. Even-numbered positions never engage with odd-numbered positions. By running the 10-cycle concurrently with a 12-cycle in length, a longer cycle of sixty pairs of characters is generated. Hours, days, months and years all follow this cycle of 60, and the cycle repeats itself endlessly.

The Chinese sexagenary cycle