Exodus 12:2

2. this month shall be unto you the beginning of months--the first not only in order but in estimation. It had formerly been the seventh according to the reckoning of the civil year, which began in September, and continued unchanged, but it was thenceforth to stand first in the national religious year which began in March, April.

Robert Jamieson, A. R. Fausset & David Brown, 1871 · public domain

Verse 2 This month shall be unto you the beginning of months - It is supposed that God now changed the commencement of the Jewish year. The month to which this verse refers, the month Abib, answers to a part of our March and April; whereas it is supposed that previously to this the year began with Tisri, which answers to a part of our September; for in this month the Jews suppose God created the world, when the earth appeared at once with all its fruits in perfection. From this circumstance the Jews have formed a twofold commencement of the year, which has given rise to a twofold denomination of the year itself, to which they afterwards attended in all their reckonings: that which began with Tisri or September was called their civil year; that which began with Abib or March was called the sacred or ecclesiastical year. As the exodus of the Israelites formed a particular era, which is referred to in Jewish reckonings down to the building of the temple, I have marked it as such in the chronology in the margin; and shall carry it down to the time in which it ceased to be acknowledged. Some very eminently learned men dispute this; and especially Houbigant, who contends with great plausibility of argument that no new commencement of the year is noted in this place; for that the year had always begun in this month, and that the words shall be, which are inserted by different versions, have nothing answering to them in the Hebrew, which he renders literally thus. Hic mensis vobis est caput mensium; hic vobis primus est anni mensis. "This month is to you the head or chief of the months; it is to you the first month of the year." And he observes farther that God only marks it thus, as is evident from the context, to show the people that this month, which was the beginning of their year, should be so designated as to point out to their posterity on what month and on what day of the month they were to celebrate the passover and the fast of unleavened bread. His words are these: "Ergo superest, et Hebr. ipso ex contextu efficitur, non hic novi ordinis annum constitui, sed eum anni mensem, qui esset primus, ideo commemorari, ut posteris constaret, quo mense, et quo die mensis paseha et azyma celebranda essent."

Adam Clarke, 1831 · public domain