91 In the first year of Darius, son of Ahasuerus, of the seed of the Medes, who hath been made king over the kingdom of the Chaldeans, 2 in the first year of his reign, I, Daniel, have understood by books the number of the years, (in that a word of Jehovah hath been unto Jeremiah the prophet,) concerning the fulfilling of the wastes of Jerusalem—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.