John Gill Commentary 2 Kings 9:29

"And in the eleventh year of Joram the son of Ahab began Ahaziah to reign over Judah." — 2 Kings 9:29 (ASV)

And in the eleventh year of Joram the son of Ahab began
Ahaziah to reign over Judah .
] In (2 Kings 8:25) , it is said to be in the twelfth year of his reign; it was at the close of the eleventh, and the beginning of the twelfth; or he began to reign with his father in the eleventh as here, and in the twelfth as there, when his father was dead.