再说flag应该是==1啊，最后一个if

#include <stdio.h>
int main() { 
    /* 定义需要计算的日期 */ 
       int year = 2088;//测试
            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 ("一年当中只有12个月哦！");break;    }
                                                sum = sum + day;         
                                                  if (year%400==0 || (year%4==0 && year%100!=0 ))    	
                                                  flag =1;    
                                                   else        
                                                   	flag = 0;   	  
                                                   	  if (flag = 1 && month>2){//此if没起到任何作用  
                                                   	        sum++;//对应year设定的值进行测试  发现year设定任意的数结果却不变只为221         
                                                   	           //所以此if是错的
                                                   	           }

                                                   	           printf("%d年%d月%d日是该年的第%d天",year,month, day,sum);   
                                                   	                   return 0;  }