今天是今年的第几天问题

#include <iostream>
using namespace std;
struct Date
{
int year;
int month;
int day;
int yearday;
};
void Day(Date *);
int main()
{
Date d1;
cout << "year:";
cin >> d1.year;
cout << "month:";
cin >> d1.month;
cout << "day:";
cin >> d1.day;
Day(&d1);
cout << d1.month << "月" << d1.day << "日是" << d1.year << "年的第" << d1.yearday << "天" << endl;
return 0;
}

void Day(Date *p)
{
int i;
int leap = 0;
int monthtable[2][12] = {
{ 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 },
{ 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 } };
if ((p->year % 400 == 0) || (p->year % 100 != 0) && (p->year % 4 == 0))
leap = 1;
p->yearday = p->day;
for (i = 0; i < p->month - 1; i++)
p->yearday += monthtable[leap][i];
}