Jamieson, Fausset and Brown on 2 Chronicles 35:19

19. In the eighteenth year of the reign Josiah was this passover kept--"It is said (2Ki 22:3) that Josiah sent Shaphan to Hilkiah in the eighth month of that year." If this statement rests upon an historical basis, all the events narrated here (at 2Ch 34:8-35:19) must have happened in about the space of five months and a half. We should then have a proof that the eighteenth year of Josiah's reign was reckoned from the autumn (compare 2Ch 29:3). "The eighth month" of the sacred year in the eighteenth year of his reign would be the second month of his eighteenth year, and the first month of the new year would be the seventh month [Bertheau].

2Ch 35:20-27. His Death.