This project will study how calendars evolved in late antique and medieval societies towards ever increasing standardization and fixation. The study of calendars has been neglected by historians as a technical curiosity; but in fact, the calendar was at the heart of ancient and medieval culture, as a structured concept of time, and as an organizing principle of social life.The history of calendars in late Antiquity and the Middle Ages was a complex social and cultural process, closely related to politics, science, and religion. The standardization and fixation of calendars was related in Antiquity to the rise of large, centralized empires in the Mediterranean and Near East, and in the Middle Ages, to the rise of the monotheistic, universalist religions of Christianity and Islam. The standardization and fixation of calendars contributed also, more widely, to the formation of a unified and universal culture in the ancient and medieval worlds.The standardization and fixation of ancient and medieval calendars will be analyzed by focusing on four, specific manifestations of this process: (1) the diffusion and standardization of the seven-day week in the Roman Empire; (2) the production of hemerologia (comparative calendar tables) in late Antiquity; (3) the use of Jewish calendar fixed cycles in medieval manuscripts; (4) the production and diffusion of monographs on the calendar by medieval Muslim, Christian, and Jewish scholars, especially al-Biruni’s Chronology of the Ancient Nations and Isaac Israeli’s Yesod Olam. Study of these four research areas will enable us to formulate a general interpretation and explanation of how and why calendars became increasingly standardized and fixed.This will be the first ever study of calendars on this scale, covering a wide range of historical periods and cultures, and involving a wide range of disciplines: social history, ancient and medieval astronomy and mathematics, study of religions, literature, epigraphy, and codicology.