Q: WHAT YEAR DID AMON 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 578 BC.
On the Hebrew calendar, sometimes called the biblical calendar, the year was 3469 AM.
2 Kings 21:19 – Amon was twenty and two years old when he began to reign, and he reigned two years in Jerusalem. And his mother's name was Meshullemeth, the daughter of Haruz of Jotbah.