The Chinese begin around 1300 BCE to use a cyclical system to count days.

The system consists of two groups of ideographs, the twelve branches and the ten stems, which are combined in couples, odd-to-odd and even to even, to form an endlessly repeating cycle of sixty units.

The system, known as the Sexagenary cycle of the Heavenly Stems and Earthly Branches, consists of two groups of ideographs, the twelve branches and the ten stems, which are combined in couples, odd to odd and even to even.