Convert date and time numbers to time_t AND specify the timezone

Question

I have the following integers:

int y, mon, d, h, min, s;

Their values are: 2012, 06, 27, 12, 47, 53 respectively. I want to represent the date time of "2012/06/27 12:47:53 UTC" if I have selected 'UTC' somewhere else in my application, or "2012/06/27 12:47:53 AEST" if I have selected 'AEST' somewhere else in my application.

I want to convert this into a time_t, and here's the code that I am current using to do so:

struct tm timeinfo;
timeinfo.tm_year = year - 1900;
timeinfo.tm_mon = mon - 1;
timeinfo.tm_mday = day;
timeinfo.tm_hour = hour;
timeinfo.tm_min = min;
timeinfo.tm_sec = sec;
//timeinfo.tm_isdst = 0; //TODO should this be set?

//TODO find POSIX or C standard way to do covert tm to time_t without in UTC instead of local time
#ifdef UNIX
return timegm(&timeinfo);
#else
return mktime(&timeinfo); //FIXME Still incorrect
#endif

So I am using a tm struct and mktime, however this is not working well, because it is always assuming my local time-zone.

What is the correct way of doing this?

So below is the solution that I have come up with so far. It basically does one of three things:

1. If UNIX, simply use timegm
2. If not UNIX
   1. Either, do math using the difference between UTC epoch and local epoch as an offset
      • Reservation: Math may be incorrect
   2. Or, set the "TZ" environment variable to UTC temporarily
      • Reservation: will trip up if/ when this code needs to be multithreaded

namespace tmUtil
{
    int const tm_yearCorrection = -1900;
    int const tm_monthCorrection = -1;
    int const tm_isdst_dontKnow = -1;

#if !defined(DEBUG_DATETIME_TIMEGM_ENVVARTZ) && !(defined(UNIX) && !defined(DEBUG_DATETIME_TIMEGM))
    static bool isLeap(int year)
    {
        return
            (year % 4) ? false
            : (year % 100) ? true
            : (year % 400) ? false
            : true;
    }

    static int daysIn(int year)
    {
        return isLeap(year) ? 366 : 365;
    }
#endif
}

time_t utc(int year, int mon, int day, int hour, int min, int sec)
{
    struct tm time = {0};
    time.tm_year = year + tmUtil::tm_yearCorrection;
    time.tm_mon = mon + tmUtil::tm_monthCorrection;
    time.tm_mday = day;
    time.tm_hour = hour;
    time.tm_min = min;
    time.tm_sec = sec;
    time.tm_isdst = tmUtil::tm_isdst_dontKnow;

    #if defined(UNIX) && !defined(DEBUG_DATETIME_TIMEGM) //TODO remove && 00
        time_t result;
        result = timegm(&time);
        return result;
    #else
        #if !defined(DEBUG_DATETIME_TIMEGM_ENVVARTZ)
            //TODO check that math is correct
            time_t fromEpochUtc = mktime(&time);

            struct tm localData;
            struct tm utcData;
            struct tm* loc = localtime_r (&fromEpochUtc, &localData);
            struct tm* utc = gmtime_r (&fromEpochUtc, &utcData);
            int utcYear = utc->tm_year - tmUtil::tm_yearCorrection;
            int gmtOff =
                (loc-> tm_sec - utc-> tm_sec)
                + (loc-> tm_min - utc-> tm_min) * 60
                + (loc->tm_hour - utc->tm_hour) * 60 * 60
                + (loc->tm_yday - utc->tm_yday) * 60 * 60 * 24
                + (loc->tm_year - utc->tm_year) * 60 * 60 * 24 * tmUtil::daysIn(utcYear);

            #ifdef UNIX
                if (loc->tm_gmtoff != gmtOff)
                {
                    StringBuilder err("loc->tm_gmtoff=", StringBuilder((int)(loc->tm_gmtoff)), " but gmtOff=", StringBuilder(gmtOff));
                    THROWEXCEPTION(err);
                }
            #endif

            int resultInt = fromEpochUtc + gmtOff;
            time_t result;
            result = (time_t)resultInt;
            return result;
        #else
            //TODO Find a way to do this without manipulating environment variables
            time_t result;
            char *tz;
            tz = getenv("TZ");
            setenv("TZ", "", 1);
            tzset();
            result = mktime(&time);
            if (tz)
                setenv("TZ", tz, 1);
            else
                unsetenv("TZ");
            tzset();
            return result;
        #endif
    #endif
}

N.B. StringBuilder is an internal class, it doesn't matter for the purposes of this question.

More info:

I know that this can be done easily using boost, et al. But this is NOT and option. I need it to be done mathematically, or using a c or c++ standard function, or combinations thereof.

timegm appears to solve this problem, however, it doesn't appear to part of the C / POSIX standard. This code currently is compiled on multiple platforms (Linux, OSX, WIndows, iOS, Android (NDK)), so I need to find a way to make it work across all of these platforms, even if the solution involves #ifdef $PLATFORM type things.

Answer 1

It makes me want to throw up in my mouth a little bit, but you could convert it to a string with strftime(), replace the timezone in the string and then convert it back with strptime() and into a time_t with mktime(). In detail:

#ifdef UGLY_HACK_VOIDS_WARRANTY
time_t convert_time(const struct tm* tm)
{
    const size_t BUF_SIZE=256;
    char buffer[BUF_SIZE];
    strftime(buffer,256,"%F %H:%M:%S %z", tm);
    strncpy(&buffer[20], "+0001", 5); // +0001 is the time-zone offset from UTC in hours
    struct tm newtime = {0};
    strptime(buffer, "%F %H:%M:%S %z", &newtime);
    return mktime(&newtime);
}
#endif

However, I would highly recommend you convince the powers that be that boost is an option after all. Boost has great support for custom timezones. There are other libraries that do this elegantly as well.