题目分析：闰年 2 月29 天，平年 2 月 28 天

代码如下：

#include <stdio.h>
int sum_day(int month,int day)        // 计算日期 
  {
  int day_tab[13]={0,31,28,31,30,31,30,31,31,30,31,30,31};
   int i;                  
   for (i=1;i<month;i++)
      day+=day_tab[i];      //累加所在月之前天数 
   return(day);
  }                         // 判断是否为闰年 
int leap(int year)
 {
 int leap;
  leap=year%4==0&&year%100!=0||year%400==0;
  return(leap);
 }
int main()
{
 int year,month,day,days;
 printf("input date(year,month,day):");
 scanf("%d,%d,%d",&year,&month,&day);
 printf("%d/%d/%d ",year,month,day);
 days=sum_day(month,day);                  
 if(leap(year)&&month>=3)                 
   days=days+1;
 printf("is the %dth day in this year.\n",days);
 return 0;
}