Convert tm of another timezone into tm of the GMT timezone-CodePudding

I'm using chrono and c 20.

I have a tm struct for EST timezone, but can't figure out how to obtain the corresponding time in the GMT zone.

Here is what I've been thinking:

tm timeInAmerica = {0};
timeInAmerica.tm_year = 75;//1975
timeInAmerica.tm_month = 0;//January
timeInAmerica.tm_mday = 31;
timeInAmerica.tm_hour = 23;
timeInAmerica.tm_minute = 23;
timeInAmerica.tm_second = 23;

auto timeZone = std::chrono::locate_zone("America/New_York");
auto sysTime = timeZone->to_sys( /*needs local_time */ );

...I don't know how to convert that tm into local_time so that I can feed it into to_sys().

I also can't figure out how to convert the returned sysTime value back into tm (which will allow me to inspect the GMT year, month, day, hour, minute).

CodePudding user response：

using namespace std::chrono;

tm timeInAmerica = {0};
timeInAmerica.tm_year = 75;//1975
timeInAmerica.tm_mon = 0;//January
timeInAmerica.tm_mday = 31;
timeInAmerica.tm_hour = 23;
timeInAmerica.tm_min = 23;
timeInAmerica.tm_sec = 23;

auto lt = local_days{year{timeInAmerica.tm_year 1900}
                    /month(timeInAmerica.tm_mon 1)
                    /timeInAmerica.tm_mday}
            hours{timeInAmerica.tm_hour}
            minutes{timeInAmerica.tm_min}
            seconds{timeInAmerica.tm_sec};

auto timeZone = locate_zone("America/New_York");
auto sysTime = timeZone->to_sys(lt);

auto sysDay = floor<days>(sysTime);
year_month_day ymd = sysDay;
hh_mm_ss hms{sysTime - sysDay};

int y = int{ymd.year()};
int m = unsigned{ymd.month()};
int d = unsigned{ymd.day()};
int h = hms.hours().count();
int M = hms.minutes().count();
int s = hms.seconds().count();

I've issued a using namespace std::chrono just to keep the verbosity down to a low roar. If you would prefer to put std::chrono:: in all the right places, that's fine too.

lt is the local_time<seconds> (or just local_seconds) needed to represent the local time. Careful on the biases (1900 and 1) when converting from a tm.

To convert sysTime back into a {year, month, day, hour, minute, second} structure, first truncate sysTime to a days-precision time_point. Then that days-precision time_point can be converted to a {year, month, day} data structure.

The time of day is simply the date_time minus the date. This can be converted into an {hours, minutes, seconds} data structure: hh_mm_ss.

Both year_month_day and hh_mm_ss have getters to get the strongly typed fields. And then each strongly typed field has conversions to integral as shown above. When converting back to a tm, don't forget about the biases (1900 and 1).