Where does this prophecy about the destruction of Israel come from?

I don't agree with the calculations thing, but I do know that the bible prophesied that 'Jerusalem will be trampled on by the nations until the appointed times of the nations are fulfilled' - Luke 21:24. And isn't it true that there has always been strife in Jerusalem for as long as we can remember? However, 'concerning that day and hour nobody knows, neither the angels of the heavens, nor the Son, but only the Father.' - Matthew 24:36 - so we cannot add it all up and make a prediction can we?