mktime() always uses local time zone and converts from UTC

/in /by

If you’re working with UTC times, you may be upset to find that mktime() always converts using your local time zone thus wrecking your UTC times.  Instead of mktime() use:

  • Linux/Posix: timegm()
  • Windows: _mkgmtime()

For example, let’s use timegm() to make a UTC time which we will pass to SolarAzEl() to calculate the Solar Elevation at that time, it must be passed a UTC time!  See this post if you are interested in SolarAzEl().

//
// Make UTC time for 10:16:00 on 5th. May 2022
// and use it to calculate the Solar Elevation
// at the time via SolarAzEl().  SolarAzEl() 
// requires a UTC time!
//
tm utc;
// tm_year is time since 1900
utc.tm_year = 2022 - 1900;
// Month is zero based, i.e. Jan is month 0, May is 4
utc.tm_mon = 5 - 1;
utc.tm_mday = 5;
utc.tm_hour = 10;
utc.tm_min = 16;
utc.tm_sec = 00;
utc.tm_isdst = 0;
// Get UTC time_t val, do not change for local time offset!
tim = timegm(&utc);	// or _mkgmtime() on windows
//
// Now calculate Solar Elevation at
// GPS coordinates 52.975, -6.0494
// at sea level using the UTC time.
//
double altitude = 0;
double Az = 0.0;
double El = 0.0;
double lat = 52.975;
double lon = -6.0494;
SolarAzEl(tim, lat, lon, 0, &Az, &El);
printf("Solar Azimuth: %f\n", Az);
printf("Solar Elevation: %f\n", El);
//