枚举年份，构造检验

#include <iostream>
using namespace std;
int tmp[13] = {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
int st, ed;
bool check(int t){
    int a[4];
    int year = t, i = 0;
    bool leap = year % 4 == 0 && year % 100 != 0 || year % 400 == 0;
    while(t){
        a[i ++ ] = t % 10;
        t /= 10;
    }    
    int month = a[0] * 10 + a[1];
    int day = a[2] * 10 + a[3];
    int all = year * 10000 + month * 100 + day;
    if(all > ed || all < st)    return false;    
    if(month != 2){
        if(month > 0 && month <= 12 && day && day <= tmp[month])    return true;
        else    return false;
    }else{
        if(tmp[month] + leap >= day)    return true;
        else    return false;
    }
}
int main(){
    cin >> st >> ed;
    int startYear, endYear;
    startYear = st / 10000;
    endYear = ed / 10000;
    int cnt = 0;
    for(int i = startYear; i <= endYear; i ++ ){
        if(check(i))    cnt ++ ;
    }
    cout << cnt << endl;
    return 0;

}

纯模拟日历

#include <iostream>

using namespace std;
int tmp[13] = {0, 31, 28, 31, 30, 31, 30,
                31, 31, 30, 31, 30, 31};
int startYear, startMonth, startDay;
int endYear, endMonth, endDay;

int nYear, nMonth, nDay;


void nextDay(int year, int month, int day){
    int flag = (year % 4 == 0 && year % 100 != 0 || year % 400 == 0);
    if(flag){
        tmp[2] = 29;
    }else{
        tmp[2] = 28;
    }

    nDay = day + 1;
    nMonth = month;
    nYear = year;
    if(nDay > tmp[month]){
        nMonth ++ ;
        nDay = 1;
    }
    if(nMonth > 12){
        nYear ++ ;
        nMonth = 1;
    }
}
bool isFit(int year, int month, int day){
    char s[8];
    s[3] = year % 10 + '0';
    s[2] = year % 100 / 10 + '0'; 
    s[1] = year % 1000 / 100 + '0';
    s[0] = year / 1000 + '0';

    s[4] = month / 10 + '0';
    s[5] = month % 10 + '0';

    s[6] = day / 10 + '0';
    s[7] = day % 10 + '0';

    bool res = true;
    for(int i = 0; i < 4; i ++ ){
        if(s[i] != s[7 - i]){
            res = false;
            break;
        }
    }
    return res;
}

int main(){
    string s;
    string e;
    cin >> s;
    cin >> e;

    startDay = (s[6] - '0') * 10 + (s[7] - '0');
    startMonth = (s[5] - '0') + (s[4] - '0') * 10;
    startYear = (s[3] - '0') + (s[2] - '0') * 10 + (s[1] - '0') * 100 + (s[0] - '0') * 1000;

    endDay = (e[6] - '0') * 10 + (e[7] - '0');
    endMonth = (e[5] - '0') + (e[4] - '0') * 10;
    endYear = (e[3] - '0') + (e[2] - '0') * 10 + (e[1] - '0') * 100 + (e[0] - '0') * 1000;

    int res = 0;
    nYear = startYear;
    nMonth = startMonth;
    nDay = startDay;

    while(true){
        if(nYear == endYear && nMonth == endMonth && nDay == endDay){
            break;
        }
        if(isFit(nYear, nMonth, nDay)){
            res ++ ;
        }
        nextDay(nYear, nMonth, nDay);
    }
    if(isFit(nYear, nMonth, nDay)){
        res ++ ;
    }
    cout << res << endl;
    return 0;
}