Since the remotest antiquity, to describe the units of time, the Chinese have used a sexagesimal cycle named tiāngān dìzhī 天干地支, or gānzhī 干支, formed by combining a decimal cycle, the (ten) Heavenly Stems, tiāngān 天干, and a duodecimal cycle, the (twelve) Earthly Branches, dìzhī 地支. We will first consider the duodecimal cycle in a linguistic and cultural perspective: its origin and its association with the twelve animals, and the spread of its use through Southeast Asia in the course of history. Then, we examine two types of decimal cycle in Thai. Finally, we propose an explanation of the origin of the sexagesimal cycle as the combination of two numbering systems: a base-ten system, and a base-twelve system.