结果为什么不对啊

#include <stdio.h>

int main()

{

int year = 2008, month = 8, day = 8;

int sum = 0;

switch(month-1)

{

case 1:

sum += 31;

case 2:

if(year%4 == 0 || year%100 != 0 && year%400 == 0 )

sum +=29;

else

sum +=28;

case 3:

sum += 31;

case 4:

sum += 30;

case 5:

sum += 31;

break;

case 6:

sum += 30;

case 7:

sum += 31;

case 8:

sum += 31;

case 9:

sum += 30;

case 10:

sum += 31;

case 11:

sum += 30;

case 12:

sum += 31;

}

sum = sum + day;

printf("%d\n",sum);

return 0;

}

老师我这个结果不对，，找不到哪里错了，，拜托帮我纠正，讲解一下。谢谢老师啊！