The waxing and waning of the moon prescribed the division of the year into twelve months, usually designated during the Exile merely by numbers instead of the old Canaanite names formerly in use. But the Babylonian Nisan (corresponding to April) is found at Neh. 2: 1 and Esther 3: 7 as the first month, and on the 14th of this month preparations for Passover began. Other Babylonian names were also adopted. See months.

In the early part of the 2nd cent. BCE the book of Jubilees, a commentary on Genesis, rejected the lunar calendar, and it is possible that this solar calendar of 364 days to a year, alternative to the official calendar, was adopted by the Dead Sea community at Qumran, where copies of part of Jubilees have been found amongst the scrolls.