A calendar is a system for arranging time units for calculating time over long periods. By convention, the day is the smallest unit of time; the measurement of fractions of a day is classified as time measurement. The generality of this definition is due to the diverse methods used to compile calendars. Although some calendars repeat astronomical cycles according to fixed rules, others are based on abstract, constantly repeating cycles of no astronomical significance. Some calendars are regulated by astronomical observations, some contain each unit accurately and redundantly, and some contain ambiguities and discontinuities. Some calendars are codified in written laws; others are transmitted through oral tradition.

The common theme of making calendars is the desire to organize time units to meet the needs and businesses of society. In addition to practical purposes, the organizational process offers a sense, how illusory, also the understanding of and time itself. Calendars thus serve as a link between humanity and the cosmos. It is no wonder that calendars held a sacred status and served as a source of social order and cultural identity. Calendars provided the basis for planning agricultural, hunting and migration cycles, for divination and prediction, and for maintaining cycles of religious and civil events. Regardless of their scientific refinement, calendars should ultimately be judged as social contracts, not as scientific treatises.