c语言判断给定日期是当年的第几天

#include <stdio.h>
int main(int argc, char *argv[])
{
    printf("%d
",day(2009,3,6));
    return 0;
}

int day(int y,int m,int d){
    int month[13]={0,31,28,31,30,31,30,31,31,30,31,30,31};
    if((y%4==0 && y%100 != 0) || (y%400 == 0)) month[2]=29;
    int sum=0,i;
    for(i=1;i<m;i++){
        sum=sum+month[i];
    }
    return (sum+d);
}