Is there any software/site where you can enter two dates in the past + hashrate, and get the result how many BTC was mined in the period between these two dates?
You mean like this:
http://www.bitcoinx.com/profit/ ? (with manual form filling for each month)
No, I've meant to do it comfortably, with some database with historic difficulty, which will extrapolate the projected difficulty for each day in the past. You enter the start and end date, and the hashrate. Optionally the power consumption, but it's easy to calculate manually since it doesn't change through time. There are no other variables, you should get how much BTC is mined in that period.