rfc3339.c

// CC0 license - No Rights Reserved.

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

int main(void)
{
    time_t now = time(NULL);
    struct tm *tm;
    int off_sign;
    int off;

    if ((tm = localtime(&now)) == NULL) {
        return -1;
    }
    off_sign = '+';
    off = (int) tm->tm_gmtoff;
    if (tm->tm_gmtoff < 0) {
        off_sign = '-';
        off = -off;
    }
    printf("%d-%d-%dT%02d:%02d:%02d%c%02d:%02d",
           tm->tm_year + 1900, tm->tm_mon + 1, tm->tm_mday,
           tm->tm_hour, tm->tm_min, tm->tm_sec,
           off_sign, off / 3600, off % 3600);
    return 0;
}

2017-5-7T03:36:50+00:00

$ date +"%A, %B %-d, %Y. %_I:%-M %p %z %Z"
Sunday, May 7, 2017. 3:37 AM +0000 UTC

$ export TZ='America/Los_Angeles'
$ ./a.out ; echo
2017-5-6T20:37:41-07:00
$ date +"%A, %B %-d, %Y. %_I:%-M %p %z %Z"
Saturday, May 6, 2017. 8:37 PM -0700 PDT

Nice work. Can you add licence (MIT?) please. I want to use it, but i can't.

Done.