91 In the first year of Darius, the son of Ahasuerus, of the seed of the Medes, who was made king over the kingdom of the Chaldaeans; 2 In the first year of his rule, I, Daniel, saw clearly from the books the number of years given by the word of the Lord to the prophet Jeremiah, in which the making waste of Jerusalem was to be complete, that is, 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.