Q: WHAT YEAR DID JOASH BEGIN HIS REIGN IN JUDAH?
A: IT DEPENDS WHAT CALENDAR YOU ARE USING!
On the Gregorian calendar (the one you might have on your wall) the year was 820 BC.
On the Hebrew calendar, sometimes called the biblical calendar, the year was 3227 AM.
2 Kings 11:4 – And the seventh year Jehoiada sent and fetched the rulers over hundreds, with the captains and the guard, and brought them to him into the house of the Lord, and made a covenant with them, and took an oath of them in the house of the Lord, and shewed them the king's son.
2 Kings 12:1 – In the seventh year of Jehu Jehoash began to reign; and forty years reigned he in Jerusalem. And his mother's name was Zibiah of Beersheba.