John Gill Commentary 1 Kings 15:1

"Now in the eighteenth year of king Jeroboam the son of Nebat began Abijam to reign over Judah." — 1 Kings 15:1 (ASV)

Now in the eighteenth year of King Jeroboam the son of Nebat
reigned Abijam over Judah .
] That is, began to reign; and by this it appears that Rehoboam was in the eighteenth year of his reign when he died, for he and Jeroboam began their reign at the same time.