Albert Barnes Commentary 2 Chronicles 34:1

"Josiah was eight years old when he began to reign; and he reigned thirty and one years in Jerusalem." — 2 Chronicles 34:1 (ASV)

Compare the parallel history in 2 Kings 22; 23:1-30. The writer here is more detailed regarding the celebration of the Passover. The only apparent discrepancy between the two narratives concerns the timing of the religious reformation; the writer of Chronicles distinctly places it before the repair of the temple, while the author of Kings places it after. The best explanation seems to be that the author of Kings departed from the chronological order, which he does not claim to follow strictly.