This book is the first comprehensive study of the lunar Macedonian calendar in two decades. The mechanics of the calendar are examined in detail, and a new approach for reconstructing the sequence of intercalary months and years is proposed which, for the first time, permits a consistent interpretation of the papyrological data of the middle Ptolemaic period. It is shown that in c. 265 BC Ptolemy II deliberately set in motion a process to realign the calendar over an extended period, which ended early in the reign of Ptolemy IV. The results have implications for the origins of the financial year, the date of the Ptolemaieia, and the history of the Canopic reform of the Egyptian calendar, among other topics. Appendices consider the nature of Macedonian intercalation and the New Year outside Egypt.