The paper discusses the units of time measurement and calendars used in ancient Mesopotamia between the end of the 4^th and the first half of the 2^nd millennium BCE. Cuneiform sources make it possible to trace the development of ideas and methods for determining the basic units of time – the day, the month, and the year. The author analyzes a sequence of calendars adopted over this period: the early Semitic, the civil lunisolar, the administrative, and the so-called schematic calendars.

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