c语言 ACM上的现实超时了应该怎么改

#include "stdio.h"
#include "conio.h"
int main()
{
int day,month,year,sum,leap;
while(scanf("%d %d %d",&year,&month,&day)!=EOF)
{

 switch(month)//先计算某月以前月份的天数
 {
     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: break;
 }
 sum = sum + day;
 if( year%400==0 || (year%4==0 && year%100==0) )
    leap =1;
 else
    leap =0;
 if(leap  ==1 && month>2)
    sum++;
 printf("%d\n",sum);
}

}

你这个，刚开始计算天数就是错的，2月确定就一直是28天么？闰年怎么办？