这题不能判断天数是否出错

#include <stdio.h>
int main()
{
 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("一年没有%d个月哦~",month );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++;
if(day>=32)
    printf("一个月没有%d天哦~",day);
else
    printf("%d年%d月%d日是该年的第%d天" ,year, month, day, sum);
return 0;
}