Q: WHEN WAS JOSIAH'S 18TH YEAR?
A: IT DEPENDS WHAT CALENDAR YOU ARE USING!
On the Gregorian calendar (the one you might have on your wall) the year was 559 BC.
On the Hebrew calendar, sometimes called the biblical calendar, the year was 3488 AM.
2 Kings 22:3-4 – 3 Now it came to pass, in the eighteenth year of King Josiah, that the king sent Shaphan the scribe, the son of Azaliah, the son of Meshullam, to the house of the Lord, saying: 4 "Go up to Hilkiah the high priest, that he may count the money which has been brought into the house of the Lord, which the doorkeepers have gathered from the people.