[Dev-C++] Class question

I am learning how to create a class and I need help.  After creating the 
class and compiling my program I get the following error.

warning C4715: 'Date::assignDate' : not all control paths return a value

This is my class.  It is supposed to validate a date entered and display the 
date on the screen.  

#include <iostream>
using namespace std;

class Date
{
    public: 
        Date();
        int assignDate(int, int, int);
        void displayDate();

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

Date::Date()
{
    month = 0;
    day   = 0;
    year  = 0;
} //end of default constructor

int Date::assignDate(int m, int d, int y)
{
    month = m;
    day   = d;
    year  = y;

    if (m >=1 && m <= 12)
    {
        if (m = 2)
            if (y % 4 == 0)
                if (d >= 1 && d <= 29)
                {
                    return m;
                    return d;
                    return y;
                }
                else if (d >= 1 && d <= 28)
                    {  
                        return m;
                        return d;
                        return y;
                    }
        else if (m == 4 || m == 6 || m == 9 || m == 11)
                if (d >= 1 && d <= 30)
                {
                    return m;
                    return d;
                    return y;
                }
              else if (m == 1 || m == 3 || m == 5 || m == 7 || m == 8 || m == 
10 || m == 11)
                        if (d >= 1 && d <= 31)
                        {
                            return m;
                            return d;
                            return y;
                        }
    }//end loop 
    else
        cout << "Month does not exist" << endl;
    //end ifs
                

} //end of assignDate function


void Date::displayDate()
    {
        cout << month << "/" << day << "/" << year; 
    } //end of displayDate function