Doubt on date manipulation using time.h

I'm studying C and thinking about a problem I came across the need to study the library time.h.

Basically, I would like to calculate the difference between two dates without needing to reinvent the wheel. For this I considered 2 hypothetical dates:

04/12/2018 12:00

04/12/2018 13:00

I hope the difference between these two dates is 1 hour (3600 seconds).

For this, I'm using the following routine:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main(void){
    time_t seconds, sec1, sec2;

    struct tm *dataTimeBegin = (struct tm *)malloc(sizeof(struct tm));
    struct tm *dataTimeEnd = (struct tm *)malloc(sizeof(struct tm));

    dataTimeBegin->tm_mday = 4;
    dataTimeBegin->tm_mon = 11;
    dataTimeBegin->tm_year = 2018 - 1900;
    dataTimeBegin->tm_hour = 12;
    dataTimeBegin->tm_min = 00;
    dataTimeBegin->tm_sec = 00;

    dataTimeEnd->tm_mday = 4;
    dataTimeEnd->tm_mon = 11;
    dataTimeEnd->tm_year = 2018 - 1900;
    dataTimeEnd->tm_hour = 13;
    dataTimeEnd->tm_min = 00;
    dataTimeEnd->tm_sec = 00;

    sec1 = mktime(dataTimeBegin);
    sec2 = mktime(dataTimeEnd);

    printf("sec1: %ld\n", sec1);
    printf("sec2: %ld\n", sec2);

    printf("difftime(sec2, sec1): %f", difftime(sec2, sec1));

    return 0;
}

Contuno, I noticed strange behavior. My output indicates:

sec1: 1543935600

sec2: 1543935600

difftime (sec2, sec1): 0.000000

Why is sec2 the same as sec1? When I do:

    dataTimeBegin->tm_mday = 4;
    dataTimeBegin->tm_mon = 11;
    dataTimeBegin->tm_year = 2018 - 1900;
    dataTimeBegin->tm_hour = 13;
    dataTimeBegin->tm_min = 00;
    dataTimeBegin->tm_sec = 00;

    dataTimeEnd->tm_mday = 4;
    dataTimeEnd->tm_mon = 11;
    dataTimeEnd->tm_year = 2018 - 1900;
    dataTimeEnd->tm_hour = 13;
    dataTimeEnd->tm_min = 00;
    dataTimeEnd->tm_sec = 00;

My output is:

sec1: 1543939200

sec2: 1543935600

difftime (sec2, sec1): -3600.000000

This behavior does not make sense to me. Am I faltering in any concept?

EDIT: I have a problem with the struct, but I do not know how to do it. I have a pointer to struct, and I want to use the pointer.

EDIT 2: I followed the recommendation of zentrunix and initialized tm_isdst and it worked fine in both versions (with the use of the pointer or without)