The reverse of mktime()

[Guillaume Dargaud]

Hello all,
in following with my thread from last week about timezone conversions, I
decided to use UTC for time references. But there's still something wrong.

I have a time string, which I use to load a struct tm and then convert to a
time_t. First problem is that mktime uses local time. I thought, OK, I'll
just use it as if it was a UTC time, no difference.
And 2nd problem, if I call localtime to convert back, I'm off by one hour
and that doesn't make sense to me:

struct tm TM={0}, *TM2;
char *Pos="20120329-111359.dat"
sscanf (Pos, "%04d%02d%02d-%02d%02d%02d",
&TM.tm_year, &TM.tm_mon, &TM.tm_mday, &TM.tm_hour, &TM.tm_min, &TM.tm_sec);
TM.tm_year-=1900;
TM.tm_mon--;

Time=mktime(&TM); // tm_hour==11
TM2 = localtime (&Time); // tm_hour==12 - Doesn't make sense

Aren't mktime and localtime converse functions ?!?
--
Guillaume Dargaud
http://www.gdargaud.net/

[Azazel]

On 2012-04-02, Guillaume Dargaud wrote:
> in following with my thread from last week about timezone conversions,
> I decided to use UTC for time references. But there's still something
> wrong.
>
> I have a time string, which I use to load a struct tm and then convert
> to a time_t. First problem is that mktime uses local time. I thought,
> OK, I'll just use it as if it was a UTC time, no difference.
> And 2nd problem, if I call localtime to convert back, I'm off by one
> hour and that doesn't make sense to me:
>
> struct tm TM={0}, *TM2;
> char *Pos="20120329-111359.dat"
> sscanf (Pos, "%04d%02d%02d-%02d%02d%02d",
> &TM.tm_year, &TM.tm_mon, &TM.tm_mday, &TM.tm_hour, &TM.tm_min,
> &TM.tm_sec);
> TM.tm_year-=1900;
> TM.tm_mon--;

TM.tm_isdst == 0

> Time=mktime(&TM); // tm_hour==11
> TM2 = localtime (&Time); // tm_hour==12 - Doesn't make sense

TM2->tm_isdst == 1

--
Az.

[Azazel]

Meant also to say that if you set TM.tm_isdst to -1 mktime should
attempt to work for itself whether DST is in effect:

[azazel@kadath:~/tmp] $ cat test.c
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

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

char *Pos="20120329-111359";

sscanf (Pos, "%04d%02d%02d-%02d%02d%02d",
&TM.tm_year,
&TM.tm_mon,
&TM.tm_mday,
&TM.tm_hour,
&TM.tm_min,
&TM.tm_sec);

TM.tm_year -= 1900;
TM.tm_mon --;

TM.tm_isdst = -1;

time_t Time = mktime (&TM);
struct tm *TM2 = localtime (&Time);

printf ("%d, %d\n", TM.tm_hour, TM2->tm_hour);
return 0;
}
[azazel@kadath:~/tmp] $ ./test
11, 11

--
Az.

[Guillaume Dargaud]

> TM.tm_isdst == 0
>
>> Time=mktime(&TM); // tm_hour==11
>> TM2 = localtime (&Time); // tm_hour==12 - Doesn't make sense
>
> TM2->tm_isdst == 1
>

Hah, OK. Then I need to change it like this:

Time=mktime(&TM);
TM2 = localtime (&Time);
if (TM2->tm_isdst==1) Time+=3600;

Thanks