struct tm inside another struct - c

Question

I'm trying to calculate the date difference between two dates, using mktime and difftime. One of the dates is a struct tm inside another struct and the other date is a struct tm inside main. mktime works fine one the date inside main, but keeps returning -1 for the date inside the struct. I think I'm overlooking something pretty obvious on how to access a struct inside a struct but I couldn't find the answer. Thanks in advance.

The code is below

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

struct consulta_cand {
    struct tm nascimento;
};

int main(void)
{
    struct consulta_cand candidato;
    time_t now;
    struct tm final2012;
    double timeprint;

    now = time(NULL);
    final2012 = *localtime(&now);
    final2012.tm_mday = 28;
    final2012.tm_mon = 10 - 1;
    final2012.tm_year = 2012 - 1900;
    final2012.tm_hour = 23;
    final2012.tm_min = 59;
    final2012.tm_sec = 59;

    timeprint = mktime(&final2012);
    printf("%.f\n", timeprint);

    candidato.nascimento = *localtime(&now);
    candidato.nascimento.tm_mday = 14;
    candidato.nascimento.tm_mon = 10 - 1;
    candidato.nascimento.tm_year = 1967 - 1900;
    candidato.nascimento.tm_hour = 0;
    candidato.nascimento.tm_min = 0;
    candidato.nascimento.tm_sec = 0;

    timeprint = mktime(&candidato.nascimento);
    printf("%.f\n", timeprint);

    return 0;
}

Answer 1

There is no problem on the way the struct is accessed. The problem is the date before 1970 which mktime() doesn't accept.

Answer 2

Are you sure it returns -1, but not simply a negative value, typical -69987600 which are the seconds from 1st January 1970 (back to) 14th October 1967?

That 1967 date is before Epoch time starts, so its Epoch representation is negativ.

Answer 3

These date functions are based on the epoch year: 1970. You're using a 1967 year.

As an aside: Dennis Ritchie wanted the epoch at 1970 so that the values (both negative and positive) would span his entire life. (Or so he once said in an interview.)