I was able to duplicate your error. Definitely peculiar. I tried several variations of your formula in an Excel template without success. I then tried using your formula in a power point template and it worked fine. So, my conclusion is that Excel is not interpreting the formula the same way that Nprinting is.
You might try creating your formula in your QV app and assign it to a variable. Then you can just drag the variable into your report as an already calculated date, thus eliminating the "mis-interpretation" of the formula by Excel.