When Did Ahaziah Become King?

2 Kings 8:25 In the twelfth year of Joram son of Ahab king of Israel, Ahaziah son of Jehoram king of Judah began to reign.

2 Kings 9:29 (In the eleventh year of Joram son of Ahab, Ahaziah had become king of Judah.)

According to P.R. House, Ahaziah only rules during 841 B.C., which is Joram's twelfth year by the northern method of counting years (2 Kgs 8:25) and the eleventh year of Joram's rule by the southern means of counting regnal years.

Reference:

House, P.R. The New American Commentary, Vol. 8, 1, 2 Kings. Nashville: Broadman & Holman Publishers, 2001.