timezone error in tNMEA0183Msg:makeTime(..)

[lsoltero]

on a linux box working with RMC

input:
$GPRMC,203253.00,A,2642.08438,N,07859.51125,W,0.065,,061220,,,A*67

return from
if ( !NMEA0183ParseRMC_nc(NMEA0183Msg, GPSTime, Latitude, Longitude, COG, SOG, DaysSince1970, Variation, &DateTime)) {
return;
}

RMC: 06/12/2020 20:32:53, 26.701406, -78.991854, 0.000000, 0.065000, 18603, 0.000000, 1607304773

timezone set on my machine is EDT (GMT-4).

the issue is that DateTime == 1607304773 corresponds to
Monday, December 7, 2020 1:32:53 AM

and not 20:32:53 06/12/20

the problem is that makeTime is defined as
static inline time_t makeTime(tmElements_t &TimeElements) { return mktime(&TimeElements); }
which uses mktime which according to the docs
"The mktime() function converts a broken-down time structure, expressed as local time, to calendar time representation. "

so it uses timezone. since GPS time is always in UTC it seems that the correct definition for makeTime should be

static inline time_t makeTime(tmElements_t &TimeElements) { return mktime(&TimeElements) - timezone; }

i.e. subtract out the timezone to get back to GMT.

[lsoltero]

or better yet...

static inline time_t makeTime(tmElements_t &TimeElements) { return timegm(&TimeElements); }

[lsoltero]

a quick grep of the code shows localtime and mktime in other places. Might be a good to review the code for timezone issues.

[ttlappalainen]

I have to check - it is not that simple. In early days Arduino tim support was, what it was. It is important that any change does not cause incompamtibility to early versions so that it would suddently change behaviour in existing installations.

[ttlappalainen]

Arduino environment does not have timezone so that can not be used. I could bring timezone to library by with:

int32_t tNMEA0183Msg::GetTimeZone() {
  tmElements_t tme;
  SetYear(tme,1970);
  SetMonth(tme,1);
  SetDay(tme,2);
  SetHour(tme,0);
  SetMin(tme,0);
  SetSec(tme,0);

  return (int32_t )makeTime(tme)-SECS_PER_DAY;
}

If mktime implementation would have timezone in future also in Arduino, this should then work. Could you test that?
As addition there could be compiler definition
#if defined(linux)||defined(__linux)||defined(linux)
return timezone;
#else
...