The Prophet is particular to set down the precise time of this wonderful and blessed vision. I call it wonderful, because of the grace manifested to the Church at such a season, when suffering captivity for their rebellion. And it is most blessed surely, for the Holy Ghost hath commissioned it with blessedness to thousands of the Lord's people in all ages. The first year of Darius corresponds to the first year of Cyrus; for Cyrus and Darius, jointly reigned after the death of Belshazzar. And it was that memorable year, in which Cyrus made proclamation for the Jews to return if they wished it, to their own country, about five hundred and thirty-six years before the coming of the Lord Jesus Christ. See Ezra 1:1, etc. Daniel was at this time taught concerning the memorable prophecy of Jeremiah. See Jeremiah 25:8-13 and Jeremiah 29:10. Daniel found, by comparing what the Prophet Jeremiah in those scriptures had said, with what was passed, and then come, that the seventy years were now expired.
Related Commentaries of Daniel 9:1
Daniel 9:1-2
1 In the first year of Darius the son of Ahasuerus, of the seed of the Medes, which was made king over the realm of the Chaldeans;
2 In the first year of his reign I Daniel understood by books the number of the years, whereof the word of the LORD came to Jeremiah the prophet, that he would accomplish seventy years in the desolations of Jerusalem.