2 Kings 8:16

And in the fifth year of Joram the son of Ahab king of Israel,
&c.] Who began his reign in the eighteenth year of Jehoshaphat, ( 2 Kings 3:1 ) .

Jehoshaphat being then king of Judah;
as he continued to be two years more; for this must be in the twenty third year of his reign, and he reigned twenty five years, ( 1 Kings 22:42 ) .

Jehoram the son of Jehoshaphat king of Judah began to reign;
according to Dr. Lightfoot ^F8, there were three beginnings of his reign; "first", when his father went with Ahab to Ramothgilead, when be was left viceroy, and afterwards his father reassumed the kingdom; the "second" time was, when Jehoshaphat went with the kings of Israel and Edom against Moab; and this is the time here respected, which was in the fifth of Joram king of Israel; and the "third" time was, at the death of his father; but knew his father was living.