Charles Ellicott Commentary Exodus 12:2

"This month shall be unto you the beginning of months: it shall be the first month of the year to you." — Exodus 12:2 (ASV)

The beginning of months. —Previously, the Hebrews had begun the year with Tisri, at or near the autumnal equinox . By doing this, they followed neither the Egyptian nor the Babylonian custom. The Egyptians began the year in June, with the first rise of the Nile; the Babylonians in Nisannu, at the vernal equinox.

It was this month which was now made, by God’s command, the first month of the Hebrew year; but it did not yet have the name Nisan: it was called Abib (Exodus 13:4), the month of “greenness.” From then on, the Hebrews had two years, a civil and a sacred one (Josephus, Ant. Jud., i. 3, § 3). The civil year began with Tisri, in the autumn, at the close of the harvest; the sacred year began with Abib (called afterwards Nisan), six months earlier. It followed that the first civil month was the seventh sacred month, and vice versa.