C++计算某日期是一年的第几天

#include <iostream>
using namespace std;
int runnian(int year);
int main()
{
int year,month,day;
int days,s=0;
int a[12]={31,28,30,31,30,31,31,30,31,30,31,31};
cout<<"请输入年月日:";
cin>>year>>month>>day;
for (int i=0;i<month-1;i++)
{
s=a[i]+s;
}
if (runnian(year)&&month>2)
days=s+day+1;
else 
days=s+day;
cout<<days<<endl;
}
int runnian(int year)
{
if(year%4==0)
{
if(year%100==0)
{
if(year%400==0)
return 1;
else
return 0;