有哪位大佬可以跟我说说这代码什么意思

#include <stdio.h>
int main()
{
    /* 定义需要计算的日期 */
    int year = 2008;
    int month = 8;
    int day = 8;
    int sum,flag;
    switch(month)
    {
        case 1:sum = 0;break;
        case 2:sum = 31;break;
        case 3:sum = 59;break;
        case 4:sum = 90;break;
        case 5:sum = 120;break;
        case 6:sum = 151;break;
        case 7:sum = 181;break;
        case 8:sum = 212;break;
        case 9:sum = 243;break;
        case 10:sum = 273;break;
        case 11:sum = 304;break;
        case 12:sum = 334;break;
        default:printf("一年只有12个月");break;
    }
    sum = sum + day;
    if(year%400 == 0 ||(year%4 == 0&&year%100 != 0))
    flag = 1;
    else
    flag = 0;
    if(flag ==1&&month>2)
      sum++;
    printf("%d年%d月%d日是该年的第%d天\n",year,month,day,sum);
    return 0;
}