Ganzhi
The 10 Heavenly Stems and 12 Earthly Branches are paired in sets of 60 to record years, months, days, and hours.
The 10 Heavenly Stems and 12 Earthly Branches are paired to produce 60 unique combinations, called the Sexagenary Cycle, used to record years, months, days, and hours.
Why 60 and not 120? Because only combinations where both stem and branch are either both yang or both yin are used — for example, Jia (yang) pairs with Zi (yang) but not with Chou (yin). The least common multiple of 10 and 12 is 60.
⚠️ The daily Ganzhi cycle has been continuously tracked from ancient times to the present without interruption. Therefore, the day pillar cannot be calculated and must be looked up in a table.