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POGeneral way to manipulate the times (between timezones) in C?
 

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  1. POGeneral way to manipulate the times (between timezones) in C?
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    <p>After writing a sample code for the question about <a href="https://stackoverflow.com/questions/1173578/converting-between-timezones-in-c">converting between timezones</a>, one of the comments to it was the need for more general method for converting from timezone A to timezone B. I was curious myself too to have a more high-level primitives for such a manipulations, so I wrote the below code.</p> <p>One drawback I see is that it constantly wiggles the TZ in the environment variables, changing the notion of 'local time'. While it seems to work (though I did not check on how it react to the DST periods, but since it's based on the Olson database, presumably it should), I was curious if anyone might have some better ideas on how to deal with this task ? </p> <pre><code>#include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; #include &lt;time.h&gt; #include &lt;sys/time.h&gt; time_t utc_now() { struct timeval tv_utc; gettimeofday(&amp;tv_utc, NULL); return tv_utc.tv_sec; } void use_tz(char *timezone) { if(timezone) { setenv("TZ", timezone, 1); } else { unsetenv("TZ"); } tzset(); } time_t utc_from_local_tm(struct tm *local_tm, char *timezone) { time_t utc; use_tz(timezone); utc = mktime(local_tm); return utc; } struct tm *local_tm_from_utc(time_t utc, char *timezone) { use_tz(timezone); return localtime(&amp;utc); } int main(int argc, char *argv[]) { struct tm *tm; struct tm tm2; time_t utc, utc2, utc3; utc = utc_now(); tm = local_tm_from_utc(utc, "Europe/Brussels"); printf("Local time in Brussels now: %s", asctime(tm)); utc2 = utc_from_local_tm(tm, "Europe/Moscow"); tm = local_tm_from_utc(utc2, "UTC"); printf("UTC time if the above was the Moscow local time: %s", asctime(tm)); memset(&amp;tm2, sizeof(tm2), 0); /* 13:00:00 on 11 dec 2010 */ tm2.tm_sec = tm2.tm_min = 0; tm2.tm_hour = 13; tm2.tm_mon = 11; tm2.tm_mday = 11; tm2.tm_year = 110; utc3 = utc_from_local_tm(&amp;tm2, "Europe/Brussels"); printf("Brussels time: %s", asctime(&amp;tm2)); tm = local_tm_from_utc(utc3, "Europe/Moscow"); printf("At 13:00:00 on 11 dec 2010 CET the time in Moscow will be: %s", asctime(tm)); exit(0); } </code></pre>
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