这类题属于比较麻烦的一种题
一年中每个月有多少天（注意闰年的判断）：

void m2days(int year) {    
    if (year % 400 == 0 || (year % 4 == 0 && year % 100 != 0))
        days[2] = 29;
    else days[2] = 28;
}
int days[15] = {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};

此外，一般第一年，第一个月，最后一年，最后一个月都是需要特殊考虑的情况

日期差值

#include <time.h>
#include <iostream>
using namespace std;
int days[15] = {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
void month2day(int year) {
    if (year % 400 == 0 || (year % 4 == 0 && year % 100 != 0))
        days[2] = 29;
    else days[2] = 28;
}
int main() {
    #ifdef SUBMIT
    freopen("in.txt", "r", stdin);
    freopen("out.txt", "w", stdout);
    long _begin_time = clock();
    #endif
    int date1, date2;
    while (cin >> date1 >> date2) {
        int y1 = date1 / 10000, m1 = date1 / 100 % 100, d1 = date1 % 100;
        int y2 = date2 / 10000, m2 = date2 / 100 % 100, d2 = date2 % 100;
        int ans = 0;
        month2day(y1);
        if (y1 == y2) {
            if (m2 == m1) {
                ans += d2 - d1;
            }
            else {
                ans += days[m1] - d1;
                for (int i = m1 + 1; i < m2; i++) {
                    ans += days[i];
                }
                ans += d2;
            }
        }
        else {
            ans += days[m1] - d1;
            for (int i = m1 + 1; i <= 12; i++) {
                ans += days[i];
            }
            for (int i = y1 + 1; i < y2; i++) {
                month2day(i);
                for (int j = 1; j <= 12; j++) {
                    ans += days[j];
                }
            }
            month2day(y2);
            for (int i = 1; i < m2; i++) {
                ans += days[i];
            }
            ans += d2;
        }
        cout << ans + 1 << endl;
    }
    #ifdef SUBMIT
    long _end_time = clock();
    printf("\n\ntime = %ld ms", _end_time - _begin_time);
    #endif
    return 0;
}

日期累加

#include <time.h>
#include <iostream>
using namespace std;
int days[15] = {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
void m2days(int y) {
    if (y % 400 == 0 || (y % 4 == 0 && y % 100 != 0))
        days[2] = 29;
    else days[2] = 28;
}
int main() {
    #ifdef SUBMIT
    freopen("in.txt", "r", stdin);
    freopen("out.txt", "w", stdout);
    long _begin_time = clock();
    #endif
    int m;
    cin >> m;
    while (m--) {
        int y, m, d, ds;
        cin >> y >> m >> d >> ds;
        int y1 = y, m1 = m, d1 = d;
        m2days(y);
        if (ds >= days[m] - d + 1) {    // 注意+1，到下一个月
            ds -= days[m] - d + 1;
            d = 1;
            m = (m + 1) % 13;    // 注意是%13
            if (m == 0) {
                m = 1;
                y++;
            }
        }
        else {
            d += ds;    
            ds = 0;
        }
        while (ds > 0) {
            m2days(y);
            int nextm = (m + 1) % 13;
            if (ds >= days[m]) {
                d = 1;
                ds -= days[m];
                m = nextm;
                if (m == 0) {
                    m = 1;
                    y++;
                }
            }
            else {
                d = ds + 1;    // 注意+1
                ds = 0;
            }
        }
        printf("%d-%02d-%02d\n", y, m, d);
    }
    #ifdef SUBMIT
    long _end_time = clock();
    printf("\n\ntime = %ld ms", _end_time - _begin_time);
    #endif
    return 0;
}