Overload conversion function +

Requirements: two realized subtracting dates, date and number of days an addition, subtraction.

#include<stdio.h>
#include<stdlib.h>
using namespace std;
class Date
{
public:
    Date(){year=0;month=0;day=0;}
    Date(int y,int m,int d){year=y;month=m;day=d;}
    friend int  operator *(Date &d1,Date&d2);     ///友元重载函数
    friend Date operator +(Date &d1,Date&d2);
    friend Date operator -(Date &d1,Date&d2);
    Date(int d)            ///把Int型的数据转化成对象的格式。
    {
        year=d/360;
        d=d%360;
        month=d/30;
        d=d%30;
        day=d;
    }
    void display();
private:
    int year,month,day;
};
Date operator+(Date &d1,Date &d2)
{
    Date d3;
    if((d1.day+d2.day)>30)
        {d3.day=d1.day+d2.day-30;
        d3.month=d1.month+d2.month+1;
        }
        else
            {d3.day=d1.day+d2.day;
            d3.month=d1.month+d2.month;
            }
        if(d3.month>12)
        {
            d3.month=d3.month-12;
            d3.year=d1.year+d2.year+1;
        }
        else
            d3.year=d1.year+d2.year;
            return d3;
}
Date operator -(Date &d1,Date &d2)
{
    Date d3;
    if((d1.day-d2.day)<0)
    {
        d3.day=30+d1.day-d2.day;
        d3.month=d1.month-d1.month-1;
    }
    else
    {
        d3.day=d1.day-d2.day;
        d3.month=d1.month-d2.month;

    }
    if((d3.month)<0)
    {
        d3.month=12+d3.month;
        d3.year=d1.year-d2.year-1;

    }
    else
    {
        d3.year=d1.year-d2.year;
    }
    return d3;
}
int operator*(Date &d1,Date&d2)
{
   int a=(d1.year-1)*360+d1.month*30+d1.day;
   int b=(d2.year-1)*360+d2.month*30+d2.day;
   if((a-b)>0)
   return a-b;
   else
    return b-a;
}
void Date:: display()
{
    cout<<year<<"/"<<month<<"/"<<day<<endl;
}
int main()
{
Date d1(2019,4,10),d2(2019,5,12),d3,d4;
cout<<"两日期相差天数：";
int d=d1*d2;
cout<<d<<endl;
cout<<"2019/4/10加上1361天后的日期为：";
d4=Date(1361);
d3=d1+d4;
d3.display();
cout<<"2019/4/10减去1361天后的日期为：";
d4=Date(1361);
d3=d1-d4;
d3.display();
    return 0;
}