Date Class, implemented
A few things I see:

The subtraction(-) operator would make more sense to return a Date object instead of an int.  This would also fit in with standard practice.  If such a function is needed, I would suggest naming a specific function for that,DiffDays perhaps.  This has the advantage being more intuitive for the user.

Instead of requiring each instance of the Date object to require the user calling Set, it would be more intuitive to overload the constructor.

It seems to me that returning a string instead of a char* would be more in fitting with a c++ program.