Perhaps the passage that causes the most confusion for modern readers is the 70 7s of Daniel 9. We need to understand two things to understand the 70 7s. First we need to understand how the Bible and non-canonical Jewish sources used numbers symbolically in relation to periods of time. Second we need to understand the full Scriptural background behind the 70 7s.
Whenever we come upon a number in the Bible referring to a number of years our tendency is to assume that those numbers are literal. Most of the time this is probably the case. Once in a while, however, that assumption can lead us down the wrong path. One example is 1 Kgs. 6:1. It claims that 480 years after the exodus Solomon started building the temple. This, in all likelihood, is a nice round twelve generations, (perhaps one generation per tribe of Israel?). Otherwise, one would have to hold to an unlikely 15th century BCE date for the exodus. (a 13th century date is more likely) The book of Judges repeatedly uses, so it seems, the number forty in a non-literal way too. In non-canonical Jewish literature from the second temple period we again run into non-literal periods of years. Interestingly for our study, Jubilees breaks down history into 490 year periods (10 jubilees). I mention these examples to suggest that there is no reason why we must understand the 70 7s as a literal period of 490 years. We have to let the text decide how to take the 70 7s, and I believe that when we look at it, we'll see that the periods are symbolic.
There are two Old Testament texts that stand in the background here. The obvious one is Jeremiah 25:1-14. Here Jeremiah prophesies that the exile will end after 70 years, a good round number probably referring to a lifetime. The exiles would go and they wouldn't be coming back, but the exile wouldn't last forever.
Leviticus 25-26 is probably a second text that gets drawn upon here in Daniel 9. Leviticus 25 begins with the laws about the sabbath year and the year of jubilee. It's clear that the Chronicler uses this text to interpret Jeremiah's prophecy in 1 Chronicles 36:15-23. The land had to lie fallow for the missed sabbath years. When you come to Leviticus 26 you have a series of rewards and curses. Interestingly, the claim that God will punish them seven fold for their sin is repeated four times. Thus here in Daniel, the seventy years of Jeremiah is reinterpreted to a seven fold punishment and is 10 jubilee cycles (10 being a number symbolizing completion).
The 7o 7s are broken into three groups the first is the 7 7s. First, notice that that is the length of one jubilee, a time of release of slaves and captives. It stands for the length of time until the Jews were allowed back in the land. However, things didn't really go that well for them while they were still in the land. The exile was still ongoing in a sense. The next period of time is the 62 7s. The big thing to see here is that the period of 62 ends in the 69th week, one less than 70 weeks and 70 is a 'perfect' number. 'The antithesis of perfection is sometimes represented by one less than the perfect number (e.g., 666 as the number of the beast in Rev. 13:18). Therefore it is appropriate that the climax of devastation comes at the end of the 69th week" (Lucas 248). The last week is a suitable period of time for evil to run its course and end. So I think that we can plausibly understand the 70 weeks of years symbolically and a time period in the very general ballpark of 490 years.
We still have to ask when the 490 year period was to reach its end? Following the suggested way of reading the numbers above, I think it's best to point to the Antiochene crisis in the 2nd century. It's the closest major event to 490 years (a period not meant to be taken literally anyways) from the fall of Jerusalem (the event prompting Jeremiah's prophecy) making that the most natural starting point. Also, I believe that the rest of the visions in Daniel also deal with the same crisis, thus making that identification make sense (at least to me).