编写一个程序，程序能计算并输出某日期在该日期所在年中是第几天

 #include<iostream>
 using namespace std;
 int mon[13] = { 0,31,28,31,30,31,30,31,31,30,31,30,31 };
 struct MYDAY
 {
 	int year, month, day;
 };
 int sumdays(const MYDAY& myday)
 {
     if (myday.year % 4 == 0 && myday.year % 100 != 0 || myday.year % 400 == 0)
     {
         mon[2] = 29;
     }
     else
     {
         mon[2] = 28;
     }
     int sum = 0;
     for (int i = 1; i < myday.month; i++)
     {
         sum += mon[i];
     }
     sum += myday.day;
     return sum;
 }
 int main()
 {
 	struct MYDAY myday, mybirthday, today;
    
 	cin >> myday.year >> myday.month >> myday.day;
     cout<<sumdays(myday);
     cout << "请输入你的生日";
     cin >> mybirthday.year >> mybirthday.month >> mybirthday.day;
     int a = sumdays(mybirthday);
     cout << "请输入今天的日期";
     cin >> today.year >> today.month >> today.day;
     int b = sumdays(today);
     cout << "从你出生到现在的天数为" << b - a;
 }