91 In the first year of Darius the son of Ahasuerus, of the seed of the Medes, who was made king over the realm of the Chaldeans, 2 in the first year of his reign, I Daniel understood by the books that the number of the years, whereof the word of Jehovah came to Jeremiah the prophet, for the accomplishment of the desolations of Jerusalem, was 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.