I too in studying Gods calendar thought the months were behind but the word clearly states God calling Nissan the month of Abib, which means Hebrew ābhībh ear of grain, hence the month when grain was fresh when the grain was green enough for roasting. Here are the scriptures
1. Exodus 13:4
This day came ye out in the month Abib.
2. Exodus 23:15
Thou shalt keep the feast of unleavened bread: (thou shalt eat unleavened bread seven days, as I commanded thee, in the time appointed of the month Abib; for in it thou camest out from Egypt: and none shall appear before me empty:)
3. Exodus 34:18
The feast of unleavened bread shalt thou keep. Seven days thou shalt eat unleavened bread, as I commanded thee, in the time of the month Abib: for in the month Abib thou camest out from Egypt.
4. Deuteronomy 16:1
Observe the month of Abib, and keep the Passover unto the LORD thy God: for in the month of Abib the LORD thy God brought thee forth out of Egypt by night.
God calls it the month of abib to show when the first month (Nissan) must be.
But it also says in Genesis 1:14 And God said, Let there be lights in the firmament of the heaven to divide the day from the night; and let them be for signs, and for seasons, and for days, and years:
So it is the lights in the sky that are to determine the days, years. Abib was to help determine the beginning of the year. The precession of the equinox brought the sign of Aries to be 1-2 months later than it used to be. So abib was to show we are not to use Aries as the beginning point. When our earth is changed back to 360 days then probably Aries will again be where it should be. I took Genesis 1:14 to be literal and searched the word and sky to see how God used the sky I knew it had to be simple and it was. God Himself set the clock in the sky. You can tell just by observation. The word shows us that the new moon was celebrated two days (1 Samuel 20:27) and the new moon does not shine for two days. I know the Jews today sight in the new moon by the sighting of the sliver of the moon but God uses the moon not shining for new moons. Look at solar eclipses they only happened on the first day of the moon not shining. Lunar eclipses only happen on the 14th or 15th day of the month on full moon. Days were precise but today because the earth has started its wobble of pole shift the days are way off. Nissan always had 30 days Iyar 29 and so on except for Cheshvan and Kislev which had either 29 or 30 days. See http://www.betemunah.org/distance.html
Also looking at Sept 20, 2017 when the Great Pyramid shows to be a day of great significance as does the star Zavijaveh which conjuncts with the sun on the first day of the new moon not shining. Also the 4 blood moons of 2014-2015 they all fall on the Jewish feasts if the months were 2 months off they wouldn’t. There are signs in the heavens showing Gods Calendar just as God said.