2 in the first year of his reign, I, Daniel, perceived in the books the number of years that, according to the word of the Lord to the prophet Jeremiah, must be fulfilled for the devastation of Jerusalem, namely, seventy years. 3 Then I turned to the Lord God, to seek an answer by prayer and supplication with fasting and sackcloth and ashes.
Matthew Henry's Commentary on Daniel 9:2-3
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.