Historical Background: Dating Lehi's departure from Jerusalem: After a period of time preaching to Jerusalem, Lehi is directed to take his family and leave. We know that the call to preach to Jerusalem came in the first year of the king Zedekiah, or 597 B.C.E. When Lehi left is not exactly clear. We do know, however, that Jerusalem was still standing when Lehi and his family left it, and indeed was still standing when they made return trips to Jerusalem. Since Jerusalem was destroyed in 586 B.C.E., we know that he left before that time.
Randall P. Spackman has written a detailed treatise on chronological issues in the Book of Mormon. Concerning the departure of Lehi he quotes 1 Nephi 7:14: "For behold, the Spirit of the Lord ceaseth soon to strive with them; for behold, they have rejected the prophets, and Jeremiah have they cast into prison. And they have sought to take away the life of my father, insomuch that they have driven him out of the land."
He then concludes: "this passage provides one of the most important clues for dating the time of Lehi's departure from Jerusalem. According to Nephi, the actions taken against Jeremiah and the other prophets were directly connected with the threats on Lehi's life. This argument was not an academic discussion between friendly brothers, but an intense debate involving life and death issues... Thus, the knowledge of Lehi's sons concerning Jeremiah's imprisonment places the escape of Ishmael's family after the time when the Egyptians invaded the land of Judah" [587 B.C.E.](Spackman "Introduction to Book of Mormon Chronology" FARMS reprint, 1993, p. 10).
Spackman also notes that when Lehi speaks to Laman and desires that righteousness might flow from Laman "like unto this river" (1 Nephi 2:5-9) that water had to have been coursing in the wadis. That would meant the rainy season, or December to February (Spackman p. 8). This information has Lehi's family departing Jerusalem close to December to February, at the end of 587 or the beginning of 586 B.C.E.