The Heavenly Stems and Earthly Branches were a system used in ancient China to mark years, months, days, and hours. In Journey to the West, the universe is divided into 12 cyclical periods, each named after the Earthly Branches. Similarly, ancient Chinese divided the day into 12 segments, also named after these branches. For example, master Subhuti taught Sun Wukong the path to immortality at midnight, called Zi hour, which spans 11 PM to 1 AM.
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