std::mktime and timezone info

Question

I'm trying to convert a time info I reveive as a UTC string to a timestamp using std::mktime in C++. My problem is that in <ctime> / <time.h> there is no function to convert to UTC; mktime will only return the timestamp as local time.

So I need to figure out the timezone offset and take it into account, but I can't find a platform-independent way that doesn't involve porting the whole code to boost::date_time. Is there some easy solution which I have overlooked?

Answer 1

timestamp = mktime(&tm) - _timezone; 

or platform independent way:

 timestamp = mktime(&tm) - timezone; 

If you look in the source of mktime() on line 00117, the time is converted to local time:

seconds += _timezone; 

Answer 2

mktime() uses tzname for detecting timezone. tzset() initializes the tzname variable from the TZ enviroment variable. If the TZ variable appears in the enviroment but its value is empty or its value cannot be correctly interpreted, UTC is used.

A portable (not threadsafe) version according to the timegm manpage

   #include <time.h>    #include <stdlib.h>     time_t    my_timegm(struct tm *tm)    {        time_t ret;        char *tz;         tz = getenv("TZ");        setenv("TZ", "", 1);        tzset();        ret = mktime(tm);        if (tz)            setenv("TZ", tz, 1);        else            unsetenv("TZ");        tzset();        return ret;    } 

Eric S Raymond has a threadsafe version published in his article Time, Clock, and Calendar Programming In C

time_t my_timegm(register struct tm * t) /* struct tm to seconds since Unix epoch */ {     register long year;     register time_t result; #define MONTHSPERYEAR   12      /* months per calendar year */     static const int cumdays[MONTHSPERYEAR] =         { 0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334 };      /*@ +matchanyintegral @*/     year = 1900 + t->tm_year + t->tm_mon / MONTHSPERYEAR;     result = (year - 1970) * 365 + cumdays[t->tm_mon % MONTHSPERYEAR];     result += (year - 1968) / 4;     result -= (year - 1900) / 100;     result += (year - 1600) / 400;     if ((year % 4) == 0 && ((year % 100) != 0 || (year % 400) == 0) &&         (t->tm_mon % MONTHSPERYEAR) < 2)         result--;     result += t->tm_mday - 1;     result *= 24;     result += t->tm_hour;     result *= 60;     result += t->tm_min;     result *= 60;     result += t->tm_sec;     if (t->tm_isdst == 1)         result -= 3600;     /*@ -matchanyintegral @*/     return (result); }