Did Ahaziah begin to reign in the eleventh or twelfth year of Joram’s reign?

Answer:

"So Joram rested with his fathers, and was buried with his fathers in the City of David. Then Ahaziah his son reigned in his place. In the twelfth year of Joram the son of Ahab, king of Israel, Ahaziah the son of Jehoram, king of Judah, began to reign. Ahaziah was twenty-two years old when he became king, and he reigned one year in Jerusalem. His mother's name was Athaliah the granddaughter of Omri, king of Israel" (II Kings 8:25-26).

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

The first passage tells us when Ahaziah gained the throne after the death of his father. But the second passage mentions when Ahaziah began to reign. Normally we would think those two events to be the same, but not necessarily. In this particular case, Joram, Ahaziah's father, dies of a protracted illness. "After all this the LORD struck him in his intestines with an incurable disease. Then it happened in the course of time, after the end of two years, that his intestines came out because of his sickness; so he died in severe pain. And his people made no burning for him, like the burning for his fathers" (II Chronicles 21:18-19). The most logical answer is that Ahaziah began to rule when his father became incapacitated by his illness during the eleventh year of Joram, king of Israel, and then took the throne when his father died during the twelfth year of Joram, king of Israel.

It is passages like these that remind us why dating ancient events are so difficult. We often assume there is no overlap between kings, but that isn't necessarily so.