The term **甲子** is formed through the systematic combination of two ancient Chinese cyclic systems: the **Ten Heavenly Stems** (天干) and the **Twelve Earthly Branches** (地支). "甲" is the first character in the Heavenly Stems sequence, while "子" is the first in the Earthly Branches sequence. By pairing the first stem with the first branch, then the second with the second, and so on in a repeating cycle, a set of 60 unique binomial combinations is generated, with "甲子" marking the inaugural pair of this 60-unit cycle. This system was historically used to sequentially number years, months, days, and hours, creating a structured calendar and chronological framework that played a central role in traditional Chinese timekeeping, historiography, and philosophical thought.