c++ how to modify a struct using a call from another function

I'm trying to understand how to write a function that can modify a struct. I want to add a day to my struct Date. The addOneDay function below doesn't work.

My goal is to addOneDay to my birthday, using a function. How do I manipulate the data in the struct to get addOneDay to work?

#include <iostream>

    using namespace std;

    struct Date
    {
        int year;
        int month;
        int day;
    };


    Date addOneDay(const Date& date);

    Date addOneDay(const Date& date)
    {
        Date rdate = date.day+1; /* <- this doesn't work */
        return rdate;
    };

void assignValues(Date& myBirthday)
{
    myBirthday.day = 27;
    myBirthday.month = 1;
    myBirthday.year = 1962;
}

main()
{
    Date x;
    assignValues(x);

    cout << x.month << "/" << x.day << "/" << x.year << endl;

    //addOneDay(x)
};