A full month. — Literally, as margin. Thenius says Shallum cannot have reigned a full month, as Zechariah 11:8 obviously refers to the three kings Zachrriah, Shallum, and Menahem.
Related Commentaries of 2 Kings 15:13
2 Kings 15:13
13 Shallum the son of Jabesh began to reign in the nine and thirtieth year of Uzziahb king of Judah; and he reigned a full month in Samaria.