When ancient Israel fell under the dominion of great empires, its calendar was radically altered. This is because in ancient societies, time and calendars were mainly controlled by political rulers. So we find that in most of the Hebrew Bible, the months of the year are only numbered and hardly ever named; but after the Babylonian exile, in the books of Zechariah, Esther, Ezra, and Nehemiah, Babylonian month names suddenly appear and become quite frequent. The Babylonian months of Nisan, Sivan, Elul, Kislev, Tevet, Shevat, and Adar are used either on their own or alongside numbered months. Nisan, in the spring, is consistently equated in these books with the ‘first month’ of Exodus (Exod 12:2); Nisan, indeed, was the first month of the Babylonian calendar.
The use of Babylonian month names, which later became standard in the Jewish calendar, is hardly surprising in the context of the postexilic period. The Babylonian calendar originated in Babylonia (southern Iraq) in the early second millennium B.C.E., spread to the rest of Mesopotamia in the late second millennium B.C.E., and then became, in the first millennium B.C.E., the official calendar of the great empires of Assyria, Babylonia, and Persia, in use across the whole Near East. The Jews under Persian rule adopted it as their own calendar, as did many other peoples in the Persian Empire.
The Jews adopted not only Babylonian month names but also the entire Babylonian calendar. This calendar was lunar, with each month beginning at the sight of a new moon. Since twelve lunar months are approximately eleven days shorter than the solar year, the Babylonian calendar was intercalated (or evened out) every two or three years by the addition of a 13th month (usually by duplicating the 12th month, Adar, and less frequently by duplicating the sixth month, Elul). This allowed the lunar system to catch up with the sun and the seasons. This calendar may have been quite similar to the original Israelite one, which was most likely also lunar; indeed, this may have helped the Jews to adopt it without qualms.
That Jews of the postexilic period were using the official, imperial calendar to determine the dates of biblical festivals is evident, at least, from the “Passover Papyrus” from Elephantine (a Jewish colony in southern Egypt). This document indicates that in 419 B.C.E., Jews at Elephantine observed the festivals of Passover and Unleavened Bread during the first month of the Babylonian calendar, Nisan, in accordance with the Pentateuch’s prescription that these festivals be observed “in the first month” of the year.
This practice presumably continued right into the Hellenistic period, when the Babylonian calendar was still largely used for official purposes by the Aramaic-speaking peoples of the Near East. But after the Jewish Hasmonean state broke off from its Hellenistic Seleucid overlords in the mid-second century B.C.E., the Jews no longer had any reason to comply with the calendar of distant Babylon, and their calendar soon acquired distinct features. Although the Babylonian month names were retained (as in the books of Maccabees), the calendar was intercalated at different times (only the month of Adar, but not Elul, would be intercalated). Still, many Babylonian features remained central to the Jewish calendar, as the Talmud later remarked: “Rabbi Hanina said: the month names came up with them [with the exiles] from Babylon” (Jerusalem Talmud, Rosh HaShanah 1:2, 56d).