Reference for II Chronicles 13:1

בִּשְׁנַ֛ת שְׁמוֹנֶ֥ה עֶשְׂרֵ֖ה לַמֶּ֣לֶךְ יָרָבְעָ֑ם וַיִּמְלֹ֥ךְ אֲבִיָּ֖ה עַל־יְהוּדָֽה׃

In the eighteenth year of king Jeroboam began Abijah to reign over Judah.

BDB