思路很明确具体解释我就不写了看代码吧

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>
using namespace std;
int ans=0;

int days[13]={0,31,28,31,30,31,30,31,31,30,31,30,31};

int check(int x)
{
    int year = x / 10000,month = x / 100 % 100,day = x % 100;

    if(month <= 0||month > 12||day <= 0) return 0; //别忘了day=0的情况

    if(day > days[month]&&month != 2) return 0;//一定不能少month！=2不然会在这一步直接返回不执行之后的

    if(month == 2)
    {
        int k;

        k = year % 4 ==0&&year % 100||year % 400 == 0;

        if(day > days[month] + k) return 0;
    }
    return 1;
}

int main()
{
    int n,m;

    scanf("%d%d",&n,&m);

    for(int i = 1000;i <10000;i ++)
    {
        int a = i,b = i;

        for(int j = 0;j < 4;j ++)
        {
            a = a * 10 + b % 10;

            b/=10;
        }

        if(a >= n&&a <= m&&check(a)) ans++;
    }

    cout<<ans<<endl;;

    return 0;
}