运行后天数出错，但是找不到哪里有问题

Question

#include<iostream>
using namespace std;
int main()
{ int day, year, month;
    int sum = 0;
    int a[13] = { 0,31,28,31,30,31,30,31,31,30,31,30,31 };
    cin >> year >> month >> day;
    for (int i = 0; i < month; i++)
    {
        sum = sum + a[i];
        sum = sum + day;
    }
    if ((year % 4 == 0 && year % 100 != 0) || (year % 400 == 0))
    {
        if (month > 2)
            sum=sum+1;
    }
    cout << sum << endl;
    return 0;
}

Answer 1

for (int i = 0; i < month; i++)
{
sum = sum + a[i];
sum = sum + day;
}
改为：

for (int i = 0; i < month; i++)
{
sum = sum + a[i];
}
sum = sum + day;