Charles Ellicott Commentary 2 Kings 15:13

"Shallum the son of Jabesh began to reign in the nine and thirtieth year of Uzziah king of Judah; and he reigned the space of a month in Samaria." — 2 Kings 15:13 (ASV)

A full month. —Literally, as in the margin. Thenius says Shallum cannot have reigned a full month, as Zechariah 11:8 obviously refers to the three kings Zechariah, Shallum, and Menahem.