John Gill's  Exposition of the Bible 

1 Kings 15:1

Now in the eighteenth year of king Jeroboam the son of Nebat reigned Abijam over Judah.

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.