John Gill Commentary 2 Kings 15:8

"In the thirty and eighth year of Azariah king of Judah did Zechariah the son of Jeroboam reign over Israel in Samaria six months." — 2 Kings 15:8 (ASV)

In the thirty eighth year of Azariah king of Judah did Zachariah the son of Jeroboam reign over Israel in Samaria six months .

Since Azariah began to reign in the twenty seventh of Jeroboam, (2 Kings 15:1) , and Jeroboam reigned forty one years, (2 Kings 14:23) , his last year must be the fifteenth or sixteenth of Azariah, in which year Zachariah must have begun to reign, had he immediately succeeded his father in the throne; there must be therefore an interregnum of twenty two years at least, which might be owing to the dissensions among the princes and people about the succession, and a dislike to Zachariah on some account; however, after all, he must reign, though but six months, to fulfil the word of the Lord, see (2 Kings 15:12) .