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.