Advancing Days

Hi, I need to write a function that will be passed two days, and the function needs to advance the day of the first date until it matches the second date. I have written all of my code until this function. I tried to break the function into different sections but I am completely lost. I know I need to increment the day until a new month, then start over with incrementing the days, but I am not sure how to do this from month to month. Any help would be greatly appreciated!!! I only got the difference function up to Month2 and tested it, but it was not working.

Code:

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//
//This program will allow the user to enter in two dates, and will return each date in the written month, day, year
//format.  For example, if 3/3/2009 is entered, March 3, 2009 will be returned.  It will also determine which date
//is earlier and which is later, and will calculate the difference in days between the two dates.

#include <iostream>
using namespace std;

class Date
{
  public:
        Date() {month=1; day=1; year=1900;};
          //default constructor; initializes the date to 1/1/1900
        Date(int m, int d, int y);
          //constructor; initializes the date that is entered in
        void set_date(int m, int d, int y);
          //allows the user to enter in the date
        void display_date();
          //prints the date with the month in word format; i.e. 1/1/2000 will print out January 1, 2000
        void advance_date();
          //advances the date by 1 day
        int get_day();
          //returns the value of the invoking day
        int get_month();
          //returns the value of the invoking month
        int get_year();
          //returns the value of the invoking year
        bool is_less_than(Date&);
          //returns true if the original invoking date is less than the one passed in, and false otherwise
        bool is_equal(Date&);
          //returns true if the original invoking date is equal to the one passed in, and false otherwise

  private:
        int month;
        int day;
        int year;
};

int difference(Date earlier_date, Date later_date);
//calculates the difference in days between the earlier_date (date1) and the later_date (date2) passed in by the user

bool is_leap(int year);
//determines if the year is a leap year

void adjust_for_leap(int days_in_month);
//if it is a leap year, adjusts February to have 29 days rather than 28

int main()
{
        Date first_date;
        Date second_date;
        Date earlier_date;
        Date later_date;
        char slash; //reads the forward slash in the date 
        int month1, day1, year1, month2, day2, year2;
        int days_apart;

        cout << "Please enter a Date in the following format: xx/xx/xxxx\n";
        cin >> month1 >> slash >> day1 >> slash >> year1;
        first_date.set_date(month1, day1, year1);

        cout << "First Date is: ";
        first_date.display_date();

        cout << "Please enter a second Date in tbe following format: xx/xx/xxxx\n";
        cin >> month2 >> slash >> day2 >> slash >> year2;
        second_date.set_date(month2, day2, year2);

        cout << "Second Date is: ";
        second_date.display_date();
       
        if (first_date.is_equal(second_date))
        {
          cout << "The first date is the same as the second date.\n";
        }
        else if (first_date.is_less_than(second_date))
        {
          earlier_date = first_date;
          later_date = second_date;
          cout << "The first date is earlier than the second date.\n";
        }
        else
        {
          earlier_date = second_date;
          later_date = first_date;
          cout << "The first date is later than the second date.\n";
        }

        difference(earlier_date, later_date);
       
        return 0;
}

int Date::get_month()
{
        return month;
}

int Date::get_day()
{
        return day;
}

int Date::get_year()
{
        return year;
}

void Date::set_date(int m, int d, int y)
{
        month = m;
        day = d;
        year = y;
        return;
}

void Date::display_date()
{
        string name_of_month[12];
       
        name_of_month[0] = "January";
        name_of_month[1] = "February";
        name_of_month[2] = "March";
        name_of_month[3] = "April";
        name_of_month[4] = "May";
        name_of_month[5] = "June";
        name_of_month[6] = "July";
        name_of_month[7] = "August";
        name_of_month[8] = "September";
        name_of_month[9] = "October";
        name_of_month[10] = "November";
        name_of_month[11] = "December";

        cout << name_of_month[month-1] << " " << day << ", " << year << endl;
        return;
}


bool Date::is_equal(Date& second_date)
{
        if ((month == second_date.month) && (day == second_date.day) && (year == second_date.year))
          return true;
        else
          return false;
}

bool Date::is_less_than(Date& second_date)
{
        if (year < second_date.year)
        {
          return true;
        }
        else if ((month < second_date.month) && (year < second_date.year) && (day < second_date.year))
        {
          return true;
        }
        else if (day < second_date.day)
        {
          return true;
        }
        else
        {
          return false;
        }
}

int difference(Date earlier_date, Date later_date)
{
      int diff_days = 0;
      int days_in_month[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};

      //if month and year are the same, and days are either equal or different
      if ((earlier_date.get_month() == later_date.get_month()) && (earlier_date.get_year() == later_date.get_year()))
        {
          while(earlier_date.get_day() < later_date.get_day())
          {   
            earlier_date.advance_date();
            diff_days++;
          }
        }

        //if year is the same but months or days are different
        if (earlier_date.get_year() == later_date.get_year())
        {
          while (earlier_date.get_month() != later_date.get_month())
          {
            if (earlier_date.get_month() == 1)
            {
            while (earlier_date.get_month() < days_in_month[0])         
            {
              earlier_date.advance_date();
              diff_days++;
            }
            }
            if (earlier_date.get_month() == 2)
            {
            while (earlier_date.get_month() < days_in_month[1])
            {
              earlier_date.advance_date();
              diff_days++;
            }
            }
          }
        }

        cout << "The two dates are " << diff_days << " days apart.\n";
}

void Date::advance_date()
{
        day++;
        return;       
}

bool is_leap(int year)
{
        if (year %4 != 0) return(false);
        else if (year %100 != 0) return(true);
        else if (year % 400 == 0) return(true);
        else return(false);
}

void adjust_for_leap(int days_in_month[])
{
        days_in_month[1] = 29;
        return;
}

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I am having trouble writing the difference(Date&, Date&) function. Thanks in advance for any help!