This book offers a study of the calendars of ancient Mesopotamia, Egypt, Persia, Greece, Rome, Gaul, and all other parts of the Mediterranean and the Near East, from the origins up to and including Jewish and Christian calendars in late Antiquity. Particular attention is given to the structure of calendars and their political context. Most ancient calendars were set and controlled by political rulers; they served as expressions of political power, as mechanisms of social control, and sometimes, on the contrary, as assertions of political independence and dissidence. Ancient calendars were very diverse, but they all shared a common history, evolving on the whole from flexible, lunar calendars to fixed, solar schemes. The Egyptian calendar played an important role in this process, most notably inspiring the institution of the Julian calendar in Rome, the forerunner of our modern Gregorian calendar. In this book it is argued that the rise of fixed calendars was not the result of scientific or technical progress, but of major political and social changes that transformed the ancient world under the great Near Eastern, Hellenistic, and Roman Empires. The institution of standard, fixed calendars served the administrative needs of these extensive empires, but also contributed to their cultural and political cohesion. This ultimately led, conversely, to late antique perceptions of calendar diversity as an expression of heresy and cause of social schism.