In the thirty first year of Asa king of Judah began Omri to reign over Israel twelve years,.... Which are to be reckoned not from the thirty first of Asa; for Ahab the son of Omri began to reign in his thirty eighth year, and so his reign would be but seven or eight years; but they are reckoned from the twenty seventh of Asa, the beginning of it, when Elah was slain by Zimri, and he died, which to the end of the thirty eight of Asa make twelve years; for the division, according to the Jewish chronology , lasted four years; Jarchi says five ; and from the beginning of that his reign is reckoned, though he did not reign over all Israel, or completely, until the thirty first of Asa, when Tibni died:six years reigned he in Tirzah; the royal city of the kings of Israel, from Jeroboam to this time, and the other six he reigned in Samaria, built by him, as in the next verse.
Seder Olam Rabba, c. 17. p. 45. So Tzemach David, par. 1. fol. 11.