Hey crazymath, I do not think there is a simple way to do that. May be someone here would be able to give you a better advice, but here is the way to think about it. Convert your date type to double type which you can directly send to C++ dll. In vba code do something like 1000*year + 10*month + day. You will get a numeric value that way you can pass an argument to C++ dll like double to double.
Then in C++ work with <ctime.h> library using tm structure and difftime method.
Good Luck