How to convert from UTC to local time in C?

Question

It\'s a simple question, but the solution appears to be far from simple. I would like to know how to convert from UTC to local time. I am looking for a solution in C that\'s

Answer 1

I followed the answer by @Dachschaden and I made an example which also shows human-readable output and I remove the DST option for the difference in seconds between UTC and local time. Here it is:

#include 
#include 

#define DATE_MAX_STR_SIZE 26
#define DATE_FMT "%FT%TZ%z"

int main() {

    time_t now_time, now_time_local;
    struct tm now_tm_utc, now_tm_local;
    char str_utc[DATE_MAX_STR_SIZE];
    char str_local[DATE_MAX_STR_SIZE];

    time(&now_time);
    gmtime_r(&now_time, &now_tm_utc);
    localtime_r(&now_time, &now_tm_local);

    /* human readable */
    strftime(str_utc, DATE_MAX_STR_SIZE, DATE_FMT, &now_tm_utc);
    strftime(str_local, DATE_MAX_STR_SIZE, DATE_FMT, &now_tm_local);

    printf("\nUTC: %s", str_utc);
    printf("\nLOCAL: %s\n", str_local);

    /* seconds (epoch) */
    /* let's forget about DST for time difference calculation */
    now_tm_local.tm_isdst = 0;
    now_tm_utc.tm_isdst = 0;
    now_time_local = now_time + (mktime(&now_tm_local) - mktime(&now_tm_utc));

    printf("\nUTC in seconds: %lu", now_time);
    printf("\nLOCAL in seconds: %lu\n", now_time_local);

    return 0;
}

Output on my machine is:

UTC: 2016-05-05T15:39:11Z-0500
LOCAL: 2016-05-05T11:39:11Z-0400

UTC in seconds: 1462462751
LOCAL in seconds: 1462448351

Note that DST is on in this case (there's a 1 hour time zone offset difference between UTC and LOCAL).