C. F. Keil and Franz Delitzsch on 2 Kings 14:1

Reign of Amaziah of Judah (cf. 2 Chron 25). - 2Ki 14:1-7. Length and spirit of his reign, and his victory over the Edomites. - 2Ki 14:1. Amaziah began to reign in the second year of Joash of Israel. Now as Joash of Israel ascended the throne, according to 2Ki 13:10, in the thirty-seventh year of Joash of Judah, the latter cannot have reigned thirty-nine full years, which might be reckoned as forty (2Ki 12:1), according to the principle of reckoning the current years as complete years, if the commencement of his reign took place a month or two before Nisan, and his death occurred a month or two after, without its being necessary to assume a regency.