1 Kings 16:23

The thirty and first year. This evidently indicates the time when Omri began his sole reign. He was first made king in the 27th year of Asa (vs. 15, 16). Five years later, inclusive reckoning (the 31st year of Asa), he began his reign.

Twelve years. This period has brought much difficulty to Biblical chronologists, yet it is comparatively simple. These 12 years cover the entire reign of Omri, not only the period of his undisputed rule, but also the period when Tibni ruled part of the land. Thus the years of Omri began in the 27th year of Asa, when the people made Omri king (vs. 15, 16), and terminated in Asa’s 38th year, when Omri was succeeded by his son Ahab (v. 29), a period of 12 years, inclusive reckoning (see p. 136).

Six years. From the period when he began his reign, till shortly after the death of Tibni. For a year after the death of his rival, Tirzah continued to be Omri’s capital.

Omri in Israel 31st year of Asa