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Groups > comp.lang.c > #93682

time functions weirdness

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Hello,

the task is simple: Convert a time string to a time_t and back. I'm not even
concerned with timezones, I want everything to be in UTC. But for some
reason it just doesn't work and I don't know why. Is it possible that
mktime()'s and gmtime()'s notions of the UNIX epoch differ by one hour?

My questions in detail:
1. Why 


dh@dotcom:~/c/gpx$ gcc -W -Wall timetest.c && ./a.out 
1. Original timestr: 1970-01-01T00:00:00Z
2. After mktime():   1970-01-01T00:00:00Z
   UNIX timestamp:   -3600
3. gmtime() =        1969-12-31T23:00:00Z
dh@dotcom:~/c/gpx$ gcc -W -Wall timetest.c && ./a.out 
1. Original timestr: 1970-08-01T00:00:00Z
2. After mktime():   1970-08-01T00:00:00Z
   UNIX timestamp:   18313200
3. gmtime() =        1970-07-31T23:00:00Z
dh@dotcom:~/c/gpx$ gcc -W -Wall timetest.c && ./a.out 
1. Original timestr: 2016-08-01T00:00:00Z
2. After mktime():   2016-08-01T01:00:00Z
   UNIX timestamp:   1470006000
3. gmtime() =        2016-07-31T23:00:00Z
dh@dotcom:~/c/gpx$

#define _XOPEN_SOURCE /* for strftime */

#include <time.h>
#include <stdio.h>
#include <locale.h>

#define FORMAT "%Y-%m-%dT%H:%M:%SZ"

int main(void) {
    struct tm tm = {0};
    time_t t;
    char buf[100];
    char *orig_time = "2016-08-01T00:00:00Z";

    setlocale(LC_ALL, "C"); /* doesn't make a difference */

    strptime(orig_time, FORMAT, &tm);
    printf("1. Original timestr: %s\n", orig_time);

    tm.tm_isdst = 0; /* doesn't matter if I set this to 1
    or to 0, or if I set it before or after mktime() */
    t = mktime(&tm);

    strftime(buf, sizeof buf, FORMAT, &tm);
    printf("2. After mktime():   %s\n", buf);
    printf("   UNIX timestamp:   %d\n", (int) t);

    strftime(buf, sizeof buf, FORMAT, gmtime(&t));
    printf("3. gmtime() =        %s\n", buf);

    return 0;
}

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time functions weirdness Robert Latest <boblatest@yahoo.com> - 2016-09-17 17:35 +0000
  Re: time functions weirdness Robert Latest <boblatest@yahoo.com> - 2016-09-17 17:38 +0000
  Re: time functions weirdness "James R. Kuyper" <jameskuyper@verizon.net> - 2016-09-17 14:26 -0400
    Re: time functions weirdness Robert Latest <boblatest@yahoo.com> - 2016-09-17 20:26 +0000
      Re: time functions weirdness "James R. Kuyper" <jameskuyper@verizon.net> - 2016-09-17 17:21 -0400
        Re: time functions weirdness Keith Thompson <kst-u@mib.org> - 2016-09-17 16:54 -0700
          Re: time functions weirdness "James R. Kuyper" <jameskuyper@verizon.net> - 2016-09-17 18:00 -0400
          Re: time functions weirdness Ian Collins <ian-news@hotmail.com> - 2016-09-18 10:45 +1200
      Re: time functions weirdness Keith Thompson <kst-u@mib.org> - 2016-09-17 17:24 -0700
        Re: time functions weirdness Robert Latest <boblatest@yahoo.com> - 2016-09-18 06:25 +0000
          Re: time functions weirdness Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-09-18 20:40 +0100
            Re: time functions weirdness supercat@casperkitty.com - 2016-09-18 16:00 -0700
  Re: time functions weirdness Keith Thompson <kst-u@mib.org> - 2016-09-17 17:04 -0700

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