91 It was the first year of the reign of Darius the Mede, the son of Ahasuerus, who became king of the Babylonians. 2 During the first year of his reign, I, Daniel, learned from reading the word of the Lord, as revealed to Jeremiah the prophet, that Jerusalem must lie desolate for seventy years.
Matthew Henry's Commentary on Daniel 9:1-2
Commentary on Daniel 9:1-3
(Read Daniel 9:1-3)
Daniel learned from the books of the prophets, especially from Jeremiah, that the desolation of Jerusalem would continue seventy years, which were drawing to a close. God's promises are to encourage our prayers, not to make them needless; and when we see the performance of them approaching, we should more earnestly plead them with God.