C++ :: Increment Member Function In Date Class

I have a class I am building called date and I've built all my functions to run and done all the necessary error checking in each function. However, my last and final function I have to write is where the # of days passed in as a parameter and then I have to increment the days by that many. And if the user does not increment any days at all in the parameter and leaves it blank, then it automatically increments by one day. I am having trouble with this. So for example if the user was to do this:

Date d1(10, 31, 1998); // Oct 31, 1998
Date d2(6, 29, 1950);// June 29, 1950

d1.Increment(); // d1 is now Nov 1, 1998
d2.Increment(5);// d2 is now July 4, 1950

The function starts out looking like this

void Date::Increment(int numDays = 1) {
}

I know I have to use a for loop to accomplish this. I just don't know how to get it to where the days passed in will go to the next month and then days passed in would go to the next year.