Hmmm. Could be a number of things, but I suspect as you do, it's the fact that Excel is not recognising them as dates which is causing the problems.
This is being complicated my end, because we use a different date notation to yourself
We use dd/mm/yyyy as opposed to mm/dd/yyyy
BUT, hopefully we can overcome that as follows.
This is quite a complex formula, but it will covert a date entered as text, in mm/dd/yyyy format into a standard Excel date which you can then reference in formulae etc.
The formula assumes that your text-date is in cell A1
Have a play with it and see if it goes any way towards solving your problem.