Why is 7 years not necessarily 7 years?
Well, for a start this is the bible's calender was it hebrew? so the days aren't quite the same as our calender, but the difference is not that great, so we can ignore this error.
The best reason I believe these are all in prophetic years and not literal is because with the events prophesied which have already taken place, as revealed in Daniel - the chronological years have differed from the prophesied years by a couple of decades or something like that, I forget. It doesn't matter, because all prophecy is about symbolism and prediction. We also have to understand the nature of biblical prophecy - that prophecy is not about predicting the future with a crystal ball precisely to within 5 seconds of the event actually taking place - that's what fortune tellers are for. For example, the Jews had all the prophecies in the world predicting the coming of Christ - but many still missed His coming. Their dogmatisim and rigid interpretations of prophecies that Jesus must come from Bethlehem was the main reason they did not accept that Christ was who He said He was. Of course they didn't factor in that Christ's hometown might be Nazareth. Their presumptions and rigid interpretation of the scriptures played a role in them rejecting Christ.
The point is Christ doesn't have to come back at exactly the last day of the 7th year in our 365 day calender because the bible says "7 years" - it's that the number 7 is symbolic and represents completion that is important. It could be 5, it could be 9, it could be 10 years, so , it is correct to say that no one knows the day or hour or year, in chronological terms. You typically find round symbolic numbers in the scriptures, like 40, 7, 3, 12 and multiples of these. That's why you never see a number like 34.5834 years - because the scripture does not give accurate chronological figures in the prophecies, nor does it have to, nor should we assume that they are.
So, my evidence that it is symbolic is mainly because the historical record has differed from the prophesied and all attempts to reconcile the two have failed. That is not my own idea however, I have consulted the contemporary writings of experts in eschatology and what methods they use, and that's all I present here.
That human beings will live no longer than 70 or 80 or 100 years , as scripture says, is another example of numbers in the bible not being precise. Obviously some people live more than 100 sometimes, 104, or 110, whatever.
I'm not saying it could not be 7 years, i'm saying it doesn't have to be 7 years. The number 7 is symbolic, as is 3.5 exactly half of 7, so, don't be surprised or claim the bible is wrong if it isn't exactly 7 years, as past historical fulfillment of prophecies have shown. You might miss the tribulation period, like the Jews missed Christ because he didnt come from Bethlehem as they thought he should. What will you do, 9 years into the tribulation period say, "gee, it's been 2 years over 7, this must not be the tribulation". ? As with all symbolism - it's not the time scale that's so important it's what it means that's important. In that regard it doesn't matter if the anti christ reigns for only 4 years and the whole tribulation period goes for 10. THe point is that a) christians endure the tribulation faithfully and to prepare for such events, and b) that this is a remarkable and wonderful event for the church which is to be, as the number 7 symbolises - the completion of all things - end game.