Re: Date handling.

[Jan Harkes <jaharkes@cs.cmu.edu>]

On Sun, Apr 24, 2005 at 04:38:49PM +1000, David Woodhouse wrote:
> On Sat, 2005-04-23 at 23:04 -0400, Jan Harkes wrote:
> > I noticed that some commit timestamps seemed to be off, looking into it
> > a bit more it seems like mktime is influenced by the setting of the
> > local TZ environment.
> 
> Ewww. I missed that in the documentation. I suppose I should have worked
> it out having empirically determined that it ignores the tm_gmtoff
> field.
> 
> > The question is, do we want to just calculate the time_t offset
> > ourselves without using mktime, or force the TZ environment to UTC.
> 
> I don't think we want to be in the business of counting leap seconds; we
> need to let the system do it. I don't much like setting TZ to UTC though
> -- how about we use your test case to find the offset and subtract that?
> 
> Does this work?

As Russ mentioned, that probably doesn't work with daylight savings
time. However I did some testing and it looks like the following lines
around mktime make it work as we would expect.

    tm.tm_isdst = -1;
    then = mktime(&tm);
    then += tm.tm_gmtoff;

Attached is the program I used to test it, it seems pretty much unfazed
by changes to the TZ environment variable. Although I tested around a
daylight savings time switch, I'm still not 100% sure if it doesn't mess
up in some corner case.

Jan


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

time_t mkutctime(struct tm *tm, int offset)
{
    time_t time;

    /* we don't know whether our timezone happens to be dst or not, let libc
     * figure that one out. */
    tm->tm_isdst = -1;

    /* interpret struct tm in the local timezone */
    time = mktime(tm);
    if (time == -1) return -1;

    /* libc lets us know how many seconds our local time differs from UTC
     * this is a non-standard BSD extension, which is probably not as
     * portable, but it seems to work. */
    time += tm->tm_gmtoff;

    /* However as the passed in struct tm was not UTC but in some other
     * timezone, we still have subtract the offset that came with the
     * RFC2822 date */
    time -= offset;

    return time;
}

int main(int argc, char **argv)
{
    struct tm tm = { 0, };
    time_t time;

    tm.tm_year = 70;
    tm.tm_mday = 1;
    time = mkutctime(&tm, 0);
    printf("1970-01-01 00:00:00 UTC = 0 (%d)\n", time);

    tm.tm_year = 105;
    tm.tm_mon = 2;
    tm.tm_mday = 17;
    tm.tm_hour = 20;
    tm.tm_min = 58;
    tm.tm_sec = 31;
    time = mkutctime(&tm, -5 * 3600);
    printf("2005-03-17 20:58:31 EST = 1111111111 (%d)\n", time);

    tm.tm_mon = 3;
    tm.tm_mday = 3;
    tm.tm_hour = 1;
    tm.tm_min = 59;
    tm.tm_sec = 59;
    time = mkutctime(&tm, -5 * 3600);
    printf("2005-04-03 01:59:59 EST = 1112511599 (%d)\n", time);

    tm.tm_hour = 3;
    tm.tm_min = 0;
    tm.tm_sec = 0;
    time = mkutctime(&tm, -4 * 3600);
    printf("2005-04-03 03:00:00 EDT = 1112511600 (%d)\n", time);
}