Rather than being an isolated, primitive body of knowledge the Jewish calendar tradition of 364 days constituted an integral part of the astronomical science of the ancient world. This tradition-attested in the Dead Sea Scrolls and in the Pseudepigrapha-stands out as a coherent, novel synthesis, representing the Jewish authors' apocalyptic worldview. The calendar is studied here both 'from within' - analyzing its textual manifestations - and 'from without' - via a comparison with ancient Mesopotamian astronomy. This analysis reveals that the calendrical realm constituted a significant case of inter-cultural borrowing, pertinent to similar such cases in ancient literature. Special attention is given to the "Book of Astronomy" (1 Enoch 72-82) and a variety of calendrical and liturgical texts from Qumran.