calculating time in C - AIX machine - c

Question

I am editing time value using a variable of type struct tm (adding some seconds to tm->tm_sec), but I am getting wrong results after doing mktime(&t).
Doing so in Linux gets me proper results, but in AIX not. What could be the problem?
#include <stdio.h>
#include <time.h>
#include <langinfo.h>
#include <locale.h>
int main ()
{
struct tm tm;
struct tm *end;
time_t t;
char str[20] = {'\0'};
//if (strptime("7 Feb 2013 01:47:30", "%d %b %Y %H:%M:%S", &tm) == NULL)
if (strptime("2012-10-17-01-07-30", "%Y-%m-%d-%H-%M-%S", &tm) == NULL)
{printf("Error\n");
}
tm.tm_sec = (tm.tm_sec + 1200);
//tm.tm_sec = 12;
//t = mktime(&tm);
//t = t + 12;
//end =localtime(&t);
strftime(str,20,"%Y %m %d %H %M %S",&tm);
printf("str is %s\n",str);
return 0;
}

Answer 1

I believe the correct answer is to use time_t, which is a large number representing the time in seconds since midnight of 1 Jan 1970. Adding arbitrary number of seconds here becomes very trivial.
I expect that if you are just adding seconds to tm->tm_sec, it overflows, and that causes the result to be incorrect. If you are unlucky, you will need to ripple your change in seconds all the way through to year (adding 5 seconds to 31 Dec 2013 23:59:56 will take you to 01 Jan 2014 00:00:01). Which of course can be done, but instead of:
t =+ 5;
you get about a dozen steps along the line of
tm.tm_sec += 5;
if (tm.tm_sec >= 60)
{
tm.tm_sec -= 60;
tm.tm_min += 1;
if (tm.tm_min >= 60)
{
... And so on ...
}
}
It gets even more interesting if you overflow the days in a month, since you then have to take into account of the number of days in each month, 28, 29, 30 or 31 depending on which month [and if it's a leap-year or not].

Answer 2

This is effectively what Mats said:
#include <stdio.h>
#include <time.h>
#include <langinfo.h>
#include <locale.h>
int main ()
{
struct tm tm;
time_t t;
char str[20] = {'\0'};
if (strptime("2012-10-17-01-07-30", "%Y-%m-%d-%H-%M-%S", &tm) == NULL) {
printf("error\n");
}
t = mktime(&tm);
t += 1200;
tm = *localtime(&t);
strftime(str,20,"%Y %m %d %H %M %S",&tm);
printf("str is %s\n",str);
return 0;
}
Produces:
cc -o t t.c && ./t
str is 2012 10 17 02 27 30