Albert Barnes Commentary Jeremiah 1:3

"It came also in the days of Jehoiakim the son of Josiah, king of Judah, unto the end of the eleventh year of Zedekiah, the son of Josiah, king of Judah, unto the carrying away of Jerusalem captive in the fifth month." — Jeremiah 1:3 (ASV)

The whole period contained in this verse is no less than 40 years and 6 months, namely, 18 years under Josiah, two periods of 11 years each under Jehoiakim and Zedekiah, and 3 months under each of the omitted kings, Jehoahaz and Jeconiah.

In the fifth month - The capture of Jerusalem took place in the fourth month, but its destruction was in the fifth month (see the marginal references), the ninth day of which was subsequently kept as a fast-day (Zechariah 7:3).