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.
Unfortunately, your best bet would probably be to calculate it manually and do some interpolation. Alternatively, a little bit of javascript could conceivably do the same with one of the calculators avaliable: grab the difficulty between each retarget, calculate that number, and add all of the results. Roundabout, but about the easiest I could come up with.