"Seventy weeks have been decreed for your people and your holy city, to finish the transgression, to make an end of sin, to make atonement for iniquity, to bring in everlasting righteous, to seal up vision and prophecy, and to anoint the most holy. So you are to know and discern that from the issuing of a decree to restore and rebuild Jerusalem until messiah the Prince there will be seven weeks and sixty-two weeks; it will be built again, with plaza and moat, even in times of distress. Then after the sixty-two weeks the Messiah will be cut off and have nothing, and the people of the prince who is to come will destroy the city and the sanctuary. And its end will come with a flood; even to the end there will be war; desolations are determined. And he will make a firm covenant with the many for one week, but in the middle of the week he will put a stop to sacrifice and grain offering; and on the wing of abominations will come one who makes desolate, even until a complete destruction, one that is decreed, is poured out on the one who makes desolate" (Daniel 9:24-27 NASB).
When Daniel read Jeremiah's prophecy (Dan. 9:2; Jere. 25:11, 12) that the Jew's captivity from Babylon was nearly over and her restoration back to Israel, to rebuild Jerusalem and the second temple, was to soon be realized, he sought God and God answered him with the vision that we've familiarly come to know as the "seventy weeks" prophecy of Daniel. Two reasons are given for the Babylonian captivity: continued idolatry, and failure to give the land 70 sabbatical years of rest (2 Chron. 36:14-21). The Babylonian captivity lasted 70 years (605-536 B.C.) evidently because this was the number of seventh-year rests for the land the Jews failed to observe. (I.e., over a period of 490 years they failed to keep the law and let the land lie fallow every seventh year; see: Lev. 25:2-7). Now that this 70 years of captivity was near completion, Daniel's "70 weeks" prophecy was about to commence with the returning of the Jews to Jerusalem and the rebuilding of their temple (see: Ezra & Nehemiah).
Below is a breakdown of the "seventy weeks" prophecy (Dan. 9:24-27) to help better understand its significance to end-times. Note that a "week" is a period of seven years, i.e., 70 x 7 (a "week") = 490 years.
The following chart reveals the accuracy of the dates both with the Jewish and the Gregorian calendars:
Remember that the Jewish calender is based on a 360 day year and not a 365.25 day year
so to make things as accurate as possible, we should take into account the extra 5.25 days we pick up each year when switching from their calendar to our modern calendar. However, we can do better than this if we express our time interval in days and not years - 360 day/year x 7 years x 69 = 173,880 days. This number is valid for either calender as demonstrated in the graph above.