Accepted systems for the division of time into days, months, and years, calendars reflect a human effort to measure and order extended periods of the future.

Scholars divide calendars into three general categories—lunar, solar, and lunisolar—depending upon whether their temporal divisions are principally based upon the movements of the moon or the sun or on a compromise between both. Regarding Western forms of chronology, in which solar reckoning of time predominates, historians have studied calendar reform, which encapsulates many scientific advancements and cosmological debates of the Renaissance and Enlightenment.

In their studies of other parts of the world, where lunar calendars tend to prevail, commentators have tended to seek a comparative understanding of the various means of envisioning and organizing time across the globe and the cultural implications of such systems.Used to regulate civil and religious observances, as well as agricultural and business affairs, calendars offer valuable insights into the historical development of cultural and scientific standards within a society.