#include<stdio.h>
#include<algorithm>
#include<string.h>
using namespace std;

int dayOfMonth[13][2]=
{
    0,0,
    31,31,
    28,29,
    31,31,
    30,30,
    31,31,
    30,30,
    31,31,
    31,31,
    30,30,
    31,31,
    30,30,
    31,31
};
struct Date
{
    int Day;
    int Month;
    int Year;
    void nextDay()
    {
        Day ++;
        if(Day>dayOfMonth[Month][ISYEAP(Year)])
        {
            Day=1;
            Month++;
            if(Month>12)
            {
                Month=1;
                Year++;
            }
        }
    }
 };
 int buf[5001][13][32];
 int abs(int x)
 {
     return x<0?-x:x;
 }
----------------------------------------------------------------------------------------------------------------------------------------------------------------
 int main()
 {
     date tmp;
     tmp.day=1;
     tmp.month=1;
     tmp.day=0;
     tmp.nextday();
     int cnt=0;
    while(tmp.year<5001)
    {
        buf[year][month][day]=cnt;
        tmp.nextday();
        cnt++ ;
     }
     while(scanf("%4d%d%d",&tmp.year,&tmp.month,&tmp.day)!=EOF)
     {
         int a=buf[tmp.year][tmp.month][tmp.day];
         scanf("%4d%d%d",&tmp.year,&tmp.month,&tmp.day);
         int b=buf[tmp.year][tmp.month][tmp.day];
         a=a-b+1;
         a=abs(a);
         printf("%d\n",a);
         
     }
     return 0;
     
     
  }
 

你是在哪里看到用结构体里定义自有函数这个用法的？
我编码实验了一下，过不了编译。

#include <stdio.h>

struct {
    int a;
    int b;
    int ab(){    //test1.c:6:10: error: expected ‘:’, ‘,’, ‘;’, ‘}’ or ‘__attribute__’ before ‘{’ token
        return 1;
    }    
} test;

int main(int argc, char **argv){
    test.a = 2 ;
    test.b = 3 ;
    printf("%d",test.ab);    //test1.c:14:21: error: ‘struct <anonymous>’ has no member named ‘ab’
    return 0;
} 

可以通过指针实现这个功能

结构体是用来定义某种类型信息的属性，调用函数能用在函数中，所以你编程有误