c++ calculating elapsed seconds from 2 time moments

I am trying to calculate how much seconds has been passed from 2 time moments. My code:

#include <iostream>
#include <ctime>

using namespace std;


int main()
{
    int FH, FM, FS, SH, SM, SS;

    cin >> FH >> FM >> FS >> SH >> SM >> SS;

    tm t1,t2;

    t1.tm_hour = FH;
    t1.tm_min = FM;
    t1.tm_sec = FS;
    t1.tm_mon = 1;
    t1.tm_mday = 1;


    t2.tm_hour = SH;
    t2.tm_min = SM;
    t2.tm_sec = SS;
    t2.tm_mon = 1;
    t2.tm_mday = 1;

    cout << difftime(mktime(&t2),mktime(&t1));
}

But it just keeps throwing 0. Input is as follows:

FH, FM, FS -> First moment hours,minutes,seconds
SH, SM, SS -> Second moments hours, minutes, seconds

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