Minimal function to format current time according to ISO8601 with milliseconds

iso8610_time.c

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

int iso8601_time(char* buf, size_t size)
{
    struct timespec ts;
    struct tm tm_local;
    struct tm tm_gm;
    clock_gettime(CLOCK_REALTIME, &ts);
    localtime_r(&ts.tv_sec, &tm_local);
    gmtime_r(&ts.tv_sec, &tm_gm);

    /* Format date and time */
    size_t len = strftime(buf, size, "%FT%T", &tm_gm);
    if (len == 0)
        return 0;

    /* Format fraction of second */
    size_t remaining = size - len;
    size_t added = snprintf(buf + len, remaining, ".%03lu", ts.tv_nsec / 1000000);
    if (added >= remaining)
        return 0;
    len += added;

    /* Format timezone offset */
    int tz_offset = (tm_local.tm_hour - tm_gm.tm_hour) * 60 + (tm_local.tm_min - tm_gm.tm_min);
    if (tz_offset) {
        char sign = tz_offset < 0 ? '-' : '+';
        int hours = (int)(abs(tz_offset)) / 60;
        int minutes = (int)(abs(tz_offset)) % 60;
        remaining = size - len;
        added = snprintf(buf + strlen(buf), remaining, "%c%02d:%02d", sign, hours, minutes);
        if (added >= remaining)
            return 0;
        len += added;
    } else {
        remaining = size - len;
        if (remaining < 2)
            return 0;
        strcpy(buf + len, "Z");
        len += 2;
    }
    return len;
}