time_until_timestamp.c

#include <stdio.h>
#include <time.h>
#include <sys/time.h>
#include <assert.h>



static time_t get_tz_offset(time_t now) {
    struct tm local, utc;
    localtime_r(&now, &local);
    gmtime_r(&now, &utc);

    time_t time_diff = local.tm_sec - utc.tm_sec +
            + (local.tm_min - utc.tm_min) * 60
            + (local.tm_hour - utc.tm_hour) * 3600;

    const int day_diff = local.tm_mday - utc.tm_mday;
    const time_t one_day = 24 * 3600;
    if (day_diff == 1) {
        // local is ahead of UTC
        time_diff += one_day;
    } else if (day_diff < -1) {
        // local is ahead of UTC and at month boundary
        time_diff += one_day;
    } else if (day_diff == -1) {
        // local is behind UTC
        time_diff -= one_day;
    } else if (day_diff > 1) {
        // local is behind UTC and at month boundary
        time_diff -= one_day;
    }
    return time_diff;
}

static time_t timestamp_to_unix_time(const char* time_str)
{
    struct tm tm_parsed;
    const char* end = strptime(time_str, "%Y-%m-%dT%H:%M:%S", &tm_parsed);
    if (end == NULL) {
        assert(false && "parse error in strptime");
    }
    int tz_hour, tz_min;
    if (sscanf(end, "%d:%d", &tz_hour, &tz_min) != 2) {
        assert(false && "parse error in sscanf");
    }
    time_t remote_tz_offset = tz_hour * 3600 + tz_min * 60;
    // At this point, tm_parsed contains local time in timezone
    // indicated by remote_tz_offset.

    // Current time
    time_t now = time(NULL);

    // offset between local time and UTC
    time_t local_tz_offset =  get_tz_offset(now);

    // difference between local and remote time zones, in seconds
    time_t total_tz_diff = local_tz_offset - remote_tz_offset;

    // adjust tm structure from remote to local timezone
    return mktime(&tm_parsed) + total_tz_diff;
}


void app_main()
{
    // set local time and timezone
    // in real application time would be set using SNTP
    setenv("TZ", "MDT-3", 1);
    tzset();
    const char* now_str = "2018-07-25T11:00:00";
    struct tm tm_now;
    strptime(now_str, "%Y-%m-%dT%H:%M:%S", &tm_now);
    time_t t_now = mktime(&tm_now);
    struct timeval tv_now = {
            .tv_sec = t_now
    };
    settimeofday(&tv_now, NULL);
  
  
    // Time stamp to get time until
    const char* time_str = "2018-07-25T11:00:00-05:00";

    printf("Time until %s: %d sec\n", time_str, (int) (timestamp_to_unix_time(time_str) - t_now));
}