Jamieson, Fausset and Brown on 1 Kings 16:23

23. In the thirty and first year of Asa ... began Omri to reign--The twelve years of his reign are computed from the beginning of his reign, which was in the twenty-seventh year of Asa's reign. He held a contested reign for four years with Tibni; and then, at the date stated in this verse, entered on a sole and peaceful reign of eight years.