程序代码：

#include<stdio.h>
struct Date
{
    int year;
    int month;
    int day;
};

int day_sum(struct Date a)
{
    int day_tab[13] = { 0,31,28,31,30,31,30,31,31,30,31,30,31 };
    int sum = 0;
    int i;
    for (i = 0; i < a.month; i++)
        sum += day_tab[i];
    sum += a.day;
    if (((a.year % 4 == 0 && a.year % 100 != 0) || a.year % 400 == 0) && a.month > 2)
        sum += 1;
    return sum;
}

int main()
{
    int day_sum(struct Date a);
    struct Date a;
    int days;
    printf("Input year,month and day:\n");
    scanf_s("%d%d%d", &a.year, &a.month, &a.day);
    days = day_sum(a);
    printf("%d/%d is the %dth day in %d\n", a.month, a.day, days, a.year);
    return 0;
}