From: "Michal Strelec" <strelec AT adam DOT osu DOT cz>
Newsgroups: comp.os.msdos.djgpp
Subject: Strange localtime function behaviour
Date: Mon, 26 Jul 1999 11:19:02 +0200
Organization: Czech Technical University
Lines: 72
Message-ID: <7nh9b2$gcb$1@ns.felk.cvut.cz>
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To: djgpp AT delorie DOT com
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Reply-To: djgpp AT delorie DOT com

Hi,
I've written small piece of code which add one (date+time) to another and
converts output to normal format (no overriding of month, days,...).

First I fill 2 variables of my custom type. rtd1 is first time and rtd2 mean
interval to be added to rtd1.

typedef struct {
  byte RTD_hour;
  byte RTD_min;
  byte RTD_sec;
  natural RTD_year;
  byte RTD_mon;
  byte RTD_day;
} RTD;

RTD rtd1,rtd2;

rtd1.RTD_hour=8;
rtd1.RTD_min=42;
rtd1.RTD_sec=23;
rtd1.RTD_year=1999;
rtd1.RTD_mon=7;
rtd1.RTD_day=26;
/* Inteval to be add to rtd1 */
rtd1.RTD_hour=0;
rtd1.RTD_min=0;
rtd1.RTD_sec=0;
rtd1.RTD_year=0;
rtd1.RTD_mon=1;
rtd1.RTD_day=0;

Now I move first time to classical time structure

    /* Now create time structure */
    cas.tm_sec=rtd1.RTD_sec;
    cas.tm_min=rtd1.RTD_min;
    cas.tm_hour=rtd1.RTD_hour;
    cas.tm_mday=rtd1.RTD_day;
    cas.tm_mon=--rtd1.RTD_mon;
    cas.tm_year=rtd1.RTD_year-1900;
    cas.tm_isdst=0; /* Must be 0 otherwise VERY nondeterministic problem may
occuare */

And add interval

    cas.tm_sec+=rtd2.RTD_sec;
    cas.tm_min+=rtd2.RTD_min;
    cas.tm_hour+=rtd2.RTD_hour;
    cas.tm_mday+=rtd2.RTD_day;
    cas.tm_mon+=rtd2.RTD_mon;
    cas.tm_year+=rtd2.RTD_year;

At last I do conversion

    /* Now convert to sec */
    nsec=mktime(&cas);
    /* And back to record */
    cas=*localtime(&nsec);

Here it is. This program works Good. But when I have ommited the line with
setting cas.tm_isdst=0;
Conversion makes sometimes strange time+dates. And I couldn't find anywhere
WHY this has to be set. cas.tm_wday or cas.tm_yday don't need to be set so
why cas.tm_isdst do?

I have tried this example in BC3.1 and there works everything fine. (even
without setting cas.isdst=0)

Mike