Convert a human readable Date to a GMT date - c

Question

I have a human readable date which is in local time. I need to convert this human readable time to a GMT time in a human readable form.
The human readable date I have is 30th March 2014 02:59.
When I convert the time to GMT I'm expecting the time to be 01:59 on the 30th but my conversion still comes out as 2:59.
I believe the only way to do the conversion is to convert the human time to an epoch and then convert this back to gmtime but doing this I still get 2:59.
Below is the code I am using:
struct tm t;
struct tm *gmtTimeStruct;
time_t t_of_day;
int year = atoi(date);
int month = atoi(date+5);
int day = atoi(date+8);
int hour = atoi(time);
int minutes = atoi(time+3);
char * gmtHumanTime = NULL;
printf("Year: %i Month: %i Day: %i Hour: %i Minutes: %i\n", year, month, day, hour, minutes);
t.tm_year = year - 1900;
t.tm_mon = month - 1;
t.tm_mday = day;
t.tm_hour = hour;
t.tm_min = minutes;
t.tm_sec = 59;
t.tm_isdst = 0;
t_of_day = mktime(&t);
printf("Epoch time: %ld\n", t_of_day);
gmtTimeStruct = gmtime(&t_of_day);
asprintf(&gmtHumanTime, "%s:%s", gmtTimeStruct->tm_hour, gmtTimeStruct->tm_min);
printf("GMT Human Time: %s", gmtHumanTime);
UPDATE 1
I've also tried change the t.tm_isdst flag from 0,1 and -1 to see if anything changes and it doesn't seem to. I know mktime has an issue with this flag as it can't always work out whether the time needs ammending due to DST or not but I'm not sure if timegm has the same issue.

Answer 1

Here's your problem:
The Daylight Saving Time flag (tm_isdst) is greater than zero if Daylight Saving Time is in effect, zero if Daylight Saving Time is not in effect, and less than zero if the information is not available.
By setting t.tm_isdst = 0; in your code, you are explicitly specifying that there is no daylight saving in effect. You should have provided a negative value.
Reference: http://www.cplusplus.com/reference/ctime/tm/
(Also, shouldn't the format string near the bottom be "%d:%02d"instead of "%s:%s"?)
EDIT
Sorry, I didn't notice you'd tried different values of tm_isdst. Perhaps it would help if you simplified your code a bit. This works perfectly on my system:
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main() {
struct tm t, *gmtTimeStruct, *localTimeStruct;
time_t t_of_day;
char *gmtHumanTime, *localHumanTime;
t.tm_year = 2014 - 1900;
t.tm_mon = 3 - 1;
t.tm_mday = 30;
t.tm_hour = 2;
t.tm_min = 59;
t.tm_sec = 59;
t.tm_isdst = -1;
t_of_day = mktime(&t);
gmtTimeStruct = gmtime(&t_of_day);
asprintf(&gmtHumanTime, "%d:%d", gmtTimeStruct->tm_hour, gmtTimeStruct->tm_min);
printf("GMT Human Time: %s\n", gmtHumanTime);
localTimeStruct = localtime(&t_of_day);
asprintf(&localHumanTime, "%d:%d", localTimeStruct->tm_hour, localTimeStruct->tm_min);
printf("Local Human Time: %s\n", localHumanTime);
return 0;
}
Output:
GMT Human Time: 1:59
Local Human Time: 2:59
Try compiling this on your system and see what you get. (Perhaps there is something wrong with your system's time zone setting?)