c++ - Incorrect time calculation with mktime to get UTC+8 -

i want current time in hong kong (utc+8), , local time utc-5.

using , running following in vs2012:

#pragma warning(disable : 4996) char buffer[10]; time_t rawtime; time(&rawtime); strftime(buffer, 10, "%h:%m:%s", localtime(&rawtime)); cout << "localtime=" << buffer << endl; strftime(buffer, 10, "%h:%m:%s", gmtime(&rawtime)); cout << "gmtime=" << buffer << endl; tm* r = gmtime(&rawtime); r->tm_hour += 8; // hong kong time mktime(r); // normalize struct strftime(buffer, 10, "%h:%m:%s", r); cout << "hongkongtime=" << buffer << endl; 

produces following output:

localtime=22:51:47 gmtime=02:51:47 hongkongtime=11:51:47 

so it's computing utc correctly, adding 8 hours producing time utc +9. what's going wrong?

and there more elegant/reliable way of getting utc+8 kludge?

you use localtime after changing tz environment variable desired timezone:

#include <iostream> #include <stdlib.h> #include <time.h>  int main(){      _putenv_s( "tz", "gmt-08:00" );      time_t mytime = time( null );     struct tm* mytm = localtime( &mytime );      std::cout << "current local time , date: " << asctime(mytm);     return 0; } 

the object mytime receive result of function time() amount of seconds since 00:00 hours, jan 1, 1970 utc, current unix timestamp. localtime() use value pointed mytime fill tm structure values represent corresponding time, expressed local timezone.

by default, timezone used localtime() 1 used in computer. however, can change function _putenv_s(), in manipulated tz variable , added new definition gmt-08:00 timezone hong kong.

in posix systems, user can specify time zone means of tz environment variable.

note more standard way of manipulating tz variable using function int setenv (const char *name, const char *value, int replace) wasn't defined in sample, used alternative.

you can read more tz environment variable here