Albert Barnes Commentary Jeremiah 1:2

"to whom the word of Jehovah came in the days of Josiah the son of Amon, king of Judah, in the thirteenth year of his reign." — Jeremiah 1:2 (ASV)

Came - literally, was ; the phrase implies that Jeremiah possessed God’s word from that time onward, not intermittently as coming and going, but constantly.

The thirteenth year of his reign - According to the ordinary reckoning, this would be 629 b.c., but if the Ptolemaic canon is right in putting the capture of Jerusalem at 586 b.c., it would be two years later, namely 627 b.c. However, according to the Assyrian chronology, it would be 608 b.c. It was the year after that in which Josiah began his reforms.