Оператор "+" не может применяться к операндам - C#

class Class1
    {
        static void Main(string[] args)
    {
        ClassDate myDate=new ClassDate();
        Console.WriteLine("default date:" + myDate.GetMonth()+ "" + myDate.GetYear());;
        ClassDate demoDate = new ClassDate(4,2013);
        Console.WriteLine("demo date:" + demoDate.GetMonth()+ "" +demoDate.GetYear());;
        ClassDate sumDate;
        int m=13;
        sumDate=myDate+demoDate;
        Console.WriteLine("sum date:" +sumDate.GetMonth()+ "" + sumDate.GetYear());
        sumDate = m + myDate;
        Console.WriteLine("sum date:" +sumDate.GetMonth()+ "" + sumDate.GetYear());
    }
    }