Q: WHAT YEAR DID ABIJAM 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 901 BC.
On the Hebrew calendar, sometimes called the biblical calendar, the year was 3146 AM.
I Kings 15:1-2 – Now in the eighteenth year of king Jeroboam the son of Nebat reigned Abijam over Judah. Three years reigned he in Jerusalem. And his mother's name was Maachah, the daughter of Abishalom.