Regardless of which regnal year system scholars seem to prefer, they all seem to agree that since God told Moses that Nisan would be the first month (Exodus 12:2), the months are numbered from Nisan, not Tishri. (Esther 3:7) "In the first month, which is the month of Nisan," (Esther 8:9) "So the king's scribes were called at that time, in the third month, which the month of Sivan," (1 Kings 6:1) "in the fourth year of Solomon’s reign over Israel, in the month Ziv, which is the second month, that he began to build the house of Yahweh." (1 Kings ) "And in the eleventh year, in the month Bul, which is the eighth month, was the house finished throughout all its parts," (1 Kings 8:2) "the feast in the month of Ethanim (Tishri), which of the ninth month, Chislev." Concerning the kings before the divided kingdom, the evidence points to the regnal year being from Nisan to Nisan. (1 Kings 6:1) "in the fourth year of Solomon’s reign over Israel, in the month Ziv, which is the second month, that he began to build the house of Yahweh." (1 Kings -38) "In the fourth year the foundation of the house of Yahweh was laid, in the month Ziv, And in the eleventh year, in the month of Bul, which is the eighth month the house was finished in all its details according to all its plans.
Until the death of Henry III in 1272, the regnal year was calculated from the day of coronation.
It would be even more complicated if Judah used a Nisan to Nisan regnal year, and the northern kingdom of Israel used a Tishri to Tishri regnal year, as the same event would be recorded by two different people as happening at different times according to their different time systems.
This is a bible study which investigates the regnal years of the Hebrew kings in scripture.
This is also known as OS (Old Style) and NS (New Style).
The Calendar Act 1752 brought about further changes.
years and lineage, the Kingdom of Judah, Sennacherib's third campaign, the religious reform, the relationship between Second Kings - and Isaiah 36-39, the messianic oracles in First Isaiah, the historical reliability of Second Chronicles 29-30 and 31-32, and Hezekiah as a second David/Solomon.