Since we cannot possibly know every nuance of Daniel's 70th week, it would be presumptuous to try and explain these various time limits. But here is how they should be seen in a general way:
70[SUP]th[/SUP] Week (less 7 months)
Then I heard one saint speaking, and another saint said unto that certain saint which spake, How long shall be the vision concerning the daily sacrifice, and the transgression of desolation, to give both the sanctuary and the host to be trodden under foot? And he said unto me, Unto two thousand and three hundred days (approx 6 years and 5 months); then shall the sanctuary be cleansed. (Dan 8:13,14)
First half of 70[SUP]th[/SUP] week + 1 month
And from the time that the daily sacrifice shall be taken away, and the abomination that maketh desolate set up, there shall be a thousand two hundred and ninety days. (3 ½ years + 30 days) (Dan 12:11)
Second half of 70[SUP]th[/SUP] week + 2 ½ months
Blessed is he that waiteth, and cometh to the thousand three hundred and five and thirty days. (3 ½ years + 75 days) (Dan 12:12)
We read "then shall the sanctuary be cleansed" which would add 75 days to the 6 years and 5 months shown above, bringing the total to about 6 years and 7 1/2 months. We know that the Lord said that the Great Tribulation would be shortened for the sake of the elect,so perhaps it is shortened by about 4 1/2 months.