Сумма и разность дат

tm t1;
    t1.tm_sec = obj1.ss;
    t1.tm_min = obj1.mm;
    t1.tm_hour = obj1.hh;
    t1.tm_mday = obj1.DD;
    t1.tm_mon = obj1.MM;
    t1.tm_year = obj1.YY - 1900;
 
    tm t2;
    t2.tm_sec = obj2.ss;
    t2.tm_min = obj2.mm;
    t2.tm_hour = obj2.hh;
    t2.tm_mday = obj2.DD;
    t2.tm_mon = obj2.MM;
    t2.tm_year = obj2.YY - 1900;
 
    time_t tt1 = mktime(&t1), tt2 = mktime(&t2);
    time_t tstr = tt2 - tt1;
    tm *tim = localtime(&tstr);

#include<ctime>
#include<iostream>
 
int main() {
        tm t1;
        t1.tm_sec = 5;
        t1.tm_min = 30;
        t1.tm_hour = 12;
        t1.tm_mday = 30;
        t1.tm_mon = 11;
        t1.tm_year = 2011 - 1900;
 
        tm t2;
        t2.tm_sec = 5;
        t2.tm_min = 30;
        t2.tm_hour = 12;
        t2.tm_mday = 30;
        t2.tm_mon = 11;
        t2.tm_year = 2011 - 1900;
 
        time_t tt1 = mktime(&t1), tt2 = mktime(&t2);
        time_t tstr = tt2 - tt1;
        tm *tim = localtime(&tstr);
 
        std::cout << tstr << '\n';
        system("pause");        return 0;
}

struct tm {
        int tm_sec;     /* seconds after the minute - [0,59] */
        int tm_min;     /* minutes after the hour - [0,59] */
        int tm_hour;    /* hours since midnight - [0,23] */
        int tm_mday;    /* day of the month - [1,31] */
        int tm_mon;     /* months since January - [0,11] */
        int tm_year;    /* years since 1900 */
        int tm_wday;    /* days since Sunday - [0,6] */
        int tm_yday;    /* days since January 1 - [0,365] */
        int tm_isdst;   /* daylight savings time flag */
        };