"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; 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." (Daniel 9:1-2)
This passage should eliminate any confusion concerning the simultaneous reigns of Darius and Cyrus. Cyrus the Persian was over the entire kingdom of the Medes and Persians while Darius reigned below him over the realm of Babylon which was the realm of the Chaldeans.
By this time in history Daniel was an old man and he had set his heart on understanding the prophecies of Jeremiah concerning the duration of Judah's punishment. Jeremiah (650-582 BC) was contemporary with Daniel (620-537 BC); therefore, Daniel was a student of his prophecies. Intensely interested in seeing the restoration of a Godfearing people, Daniel set out to understand the commencement and duration of this seventy year desolation. Concerning these things Jeremiah wrote:
And this whole land shall be a desolation, and an astonishment; and these nations shall serve the king of Babylon seventy years. And it shall come to pass, when seventy years are accomplished, that I will punish the king of Babylon, and that nation, saith the LORD, for their iniquity, and the land of the Chaldeans, and will make it perpetual desolations ... For thus saith the LORD, That after seventy years be accomplished at Babylon I will visit you, and perform my good word toward you, in causing you to return to this place (Jeremiah 25:11-12, 29:10).
The passage states that Daniel understood these things by utilizing books. These books probably comprised the prophecies of Jeremiah and the historical records of Babylon and Persia. Daniel was taken captive in 605 BC; therefore, he was likely calculating the seventy years from this date. If this was the case, the seventy years would have been accomplished in 535 BC. 539 BC was the first year of Darius, so one may easily understand how Daniel's soul was burdened for the restoration of his people.
There is considerable disagreement as to when the seventy year period began and when it ended. Some hold the view that it began in 605 BC when Daniel was taken and that it ended in 535 BC when the first wave of Israelites returned under the leadership of Jeshua and Zerubbabel; however, this author holds to the view that it began in 586 BC when Jerusalem was smitten by Nebuchadnezzar and that it ended in 516 BC when the second temple was completed under the leadership of Jeshua and Zerubbabel. Since the temple was such a significant symbol of Israel's relationship with God, its destruction and reconstruction seem to be more probable landmarks for calculating such a significant time frame. Regardless of which view is correct, Daniel was perfectly justified in being concerned for the return of his people. He correctly understood that their time was at hand and he wanted them to be spiritually prepared.
Daniel was a man of great awareness. He did not live in a self-righteous bubble of seclusion. He was not self-centered or strictly preoccupied with his needs. Much like the apostle Paul he was continuously focused on the broader will of God. Believers would do well to follow Daniel's example. It is good to be concerned with God's Church as a whole and not to live in a state of self-absorption. In the eyes of men Daniel had many reasons to look out for no one but himself but not one of those reasons kept him from having a heart that was bent on the ultimate glorification and vindication of God's righteous name.