Sexagenary Cycle (六十甲子) Explained

The Sexagenary Cycle, also known as the Sixty Jiazi (六十甲子), is an ancient Chinese calendrical system used to represent the passage of time. It is a combination of Heavenly Stems (天干) and Earthly Branches (地支), forming a 60 cycle widely used. Each year, month, day, hour or other time unit is assigned a pair from the cycle.

FIVE-MOUSE-DUN 五鼠遁

On DAYs of ^S0甲_⊕WOOD and ^S5己_⊖EARTH, HOURs start with ^00S0B0甲子.
On DAYs of ^S1乙_⊖WOOD and ^S6庚_⊕METAL, HOURs start with ^12S2B0丙子.
On DAYs of ^S2丙_⊕FIRE and ^S7辛_⊖METAL, HOURs start with ^24S4B0戊子.
On DAYs of ^S3丁_⊖FIRE and ^S8壬_⊕WATER, HOURs start with ^36S6B0庚子.
On DAYs of ^S4戊_⊕EARTH and ^S9癸_⊖WATER, HOURs start with ^48S8B0壬子.

Kaoyuan (Investigation of Origins) _ctext states: According to the Shiji (Records of the Grand Historian), Tianguan Shu (Book of Celestial Officials) _ctext: On a day of ^00S0B0甲子, the HOUR starts from ^00S0B0甲子. Counting forward from ^00S0B0甲子 in sequence to the HOUR of ^B0子_NORTH of the next day, it corresponds to ^12S2B0丙子. Therefore, on a day of ^S1乙_⊖WOOD, HOUR starts from ^12S2B0丙子. Continuing the count from day of ^S0甲_⊕WOOD to day of ^S5己_⊖EARTH, spans five days, covering a total of 60 (double) HOUR (one full cycle of the SEXAGENARY-CYCLE). This cyclical system restarts upon completion. Thus, on a day of ^S5己_⊖EARTH, the HOUR of ^B0子_NORTH also corresponds to ^00S0B0甲子.

FIVE-TIGER-DUN 五虎遁

In YEARs of ^S0甲_⊕WOOD and ^S5己_⊖EARTH, MONTHs start with ^02S2B2丙寅
In YEARs of ^S1乙_⊖WOOD and ^S6庚_⊕METAL, MONTHs starts with ^14S2B0戊寅
In YEARs of ^S2丙_⊕FIRE and ^S7辛_⊖METAL, MONTHs starts with ^26S4B0庚寅
In YEARs of ^S3丁_⊖FIRE and ^S8壬_⊕WATER, MONTHs starts with ^38S6B0壬寅
In YEARs of ^S4戊_⊕EARTH and ^S9癸_⊖WATER, MONTHs starts with ^50S8B0甲寅

Kaoyuan (Investigation of Origins) _ctext states: According to ancient calendars, the starting points of YEAR, MONTH, DAY, and HOUR were all based on ^00S0B0甲子. In a ^00S0B0甲子 YEAR, the ^00S0B0甲子 MONTH corresponds to the 11th MONTH of Winter Solstice of the previous YEAR. Since the 1st MONTH is established as ^B2寅_E30N, it corresponds to ^02S2B2丙寅. The 2nd MONTH corresponds to ^03S3B3丁卯. The cycle proceeds sequentially. By the 1st MONTH of the following year, it reaches ^14S2B0戊寅. Therefore, in YEARs of ^S1乙_⊖WOOD, the 1st month starts with ^14S2B0戊寅. Counting forward from ^S0甲_⊕WOOD to ^S5己_⊖EARTH, spans five years, and covers 60 MONTHs (one full cycle of the SEXAGENARY-CYCLE). After completing one cycle, it restarts. Thus, in YEARs of ^S5己_⊖EARTH, the 1st MONTH also corresponds to ^02S2B2丙寅.