程序代码：

#include<stdio.h>

//获取公历年初至某整月的天数
int year_sumday(int year, int month) {
    int sum = 0;
    int run[12] = {31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
    int ping[12] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
    int index, runflag = 0;

    if((year % 4 == 0 && year % 100 != 0) || year % 400 == 0) {
        runflag = 1;
    }

    for(index = 0; index < month - 1; index++) {
        if(runflag == 1) {
            sum += run[index];

        } else {
            sum += ping[index];
        }
    }

    return sum;
}

int main() {
    int y, m, d, days;
    printf("请输入年月日(请用空格隔开数据,例如2000 01 01)：");
    scanf("%d%d%d", &y, &m, &d);
    days = year_sumday(y, m) + d;
    printf("%d年1月1日 至 %d年%d月%d日  共有%d天.\n", y, y, m, d, days);
    return 0;
}