“Now it came to pass in the third year of Hoshea son of Elah king of Israel, that Hezekiah the son of Ahaz king of Judah began to reign.”
King James Version (KJV)
18:2 To reign - It is not certain that Ahaz lived only thirty six years, for those sixteen years which he reigned, may be computed, not from the first beginning of his reign, when he reigned with his father; which was at the twentieth year of his age, but from the beginning of his reigning alone.