By “the first month” is meant (compare 2 Chronicles 30:2-3) the month of Nisan, the first of the Jewish sacred year, not necessarily the first month of Hezekiah’s reign.
Related Commentaries of 2 Chronicles 29:3
2 Chronicles 29:3
3 He in the first year of his reign, in the first month, opened the doors of the house of the LORD, and repaired them.