Albert Barnes Commentary 2 Chronicles 29:3

"He in the first year of his reign, in the first month, opened the doors of the house of Jehovah, and repaired them." — 2 Chronicles 29:3 (ASV)

By “the first month,” it is meant the month of Nisan, the first of the Jewish sacred year (Compare 2 Chronicles 30:2–3), not necessarily the first month of Hezekiah’s reign.