Conversion of UTC Date and Time to UNIX Time gives wrong (time zone different) value. Why?

Question

I wrote a small function to convert Date & Time in UTC to UNIX time (Epoch time). The value that I get, however, is dependent on the time-zone I am in.

Here is the code

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

time_t GenerateUnixTimeStampFromDateAndTime(char *DateAndTime);

void main()
{
    long int UnixTime=0;
    char *CurrentTime="01/22/2019 06:30:00";

    UnixTime = (long int)GenerateUnixTimeStampFromDateAndTime(CurrentTime);
    printf("Current Unix Time= %ld\r\n", UnixTime);
}

time_t GenerateUnixTimeStampFromDateAndTime(char *DateAndTime) 
{
    struct tm ti={0};
    if( sscanf(DateAndTime, "%d/%d/%d %d:%d:%d", &ti.tm_mon, &ti.tm_mday, &ti.tm_year, &ti.tm_hour, &ti.tm_min, &ti.tm_sec) != 6 )
        return -1;
    ti.tm_year = ti.tm_year - 1900;
    ti.tm_mon = ti.tm_mon - 1;

    return mktime(&ti);
}

The answer I get is 1548118800 which is 01/22/2019 01:00:00 ie -5:30 which is the time-zone I am in (India). If I change my PC timezone to UTC, then it gives correct value of 1548138600.

What changes do i need to make it time-zone independent?

Answer 1

As documented, mktime() takes the broken-down time components in local time.

First, save the users current timezone:

char  *old_timezone, *temp;

temp = getenv("TZ");
if (temp) {
    const size_t  len = temp;
    old_timezone = malloc(len + 1);
    if (!old_timezone) {
        fprintf(stderr, "Out of memory!\n");
        exit(EXIT_FAILURE);
    }
    if (len > 0)
        memcpy(old_timezone, temp, len);
    old_timezone[len] = '\0';
} else
    old_timezone = NULL;

If the user uses the system default, old_timezone will be NULL.

Next, set the current timezone (for this process) to UTC:

setenv("TZ", "UTC", 1);
tzset();

Note that if you want, you can use any timezone specifier instead of "UTC" above; see tzset() for details. The tzset() call is usually done internally by your C library, but doing it explicitly here helps us humans see that something timezone-specific just occurred.

At this point, mktime() will operate in UTC, and localtime() and gmtime() will return the same results.

Afterwards, restore the timezone by

if (old_timezone) {
    setenv("TZ", old_timezone, 1);
    free(old_timezone);
    old_timezone = NULL;
} else
    unsetenv("TZ");
tzset();

Note that this does not affect anything except the current process (and any child processes you might create via popen() or system() or fork() and exec()). Timezone, like locale, is a per-process property.

---

Running

    unsetenv("TZ");
    tzset();

will change the current timezone for this process to the system default timezone.

---

If your program operates explicitly in UTC, you can do just

    /* This program works explicitly in the UTC timezone.
       User/system timezone configuration is completely ignored. */
    setenv("TZ", "UTC", 1");
    tzset();

near the beginning of your main().