mktime should normalize time to the next month if mday is greater than the last
day of the current month.

CTIME(3)
       The mktime() function converts a broken-down time structure, expressed
       as  local time, to calendar time representation.  The function ignores
       the specified contents of the structure members  tm_wday  and  tm_yday
       and recomputes them from the other information in the broken-down time
       structure.  If structure members are  outside  their  legal  interval,
       they  will  be normalized (so that, e.g., 40 October is changed into 9
       November).  Calling mktime() also sets the  external  variable  tzname
       with  information  about the current time zone.  If the specified bro-
       ken-down time cannot be represented as calendar  time  (seconds  since
       the  epoch),  mktime()  returns  a  value of (time_t)(-1) and does not
       alter the tm_wday and tm_yday members of the broken-down  time  struc-
       ture.


But mktime normalizes wrong, it just delete the last digits until mday is in
legal range, example:


$ cat mktime-test.c
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int main(void)
{
        char str_mday[3] = "00\0";
        char date[11] = "00.00.0000\0";
        int int_mday = 0;

        time_t t = time(NULL);
        struct tm *inctime = localtime(&t);

        strftime(str_mday, 3, "%e", inctime);
        int_mday = atoi(str_mday);

        strftime(date, 11, "%e.%m.%Y", inctime);
        printf("\ndate: %s\tmday= %d",date,int_mday);

        int_mday+=29;

        sprintf(str_mday, "%d",int_mday);
        strptime(str_mday, "%e", inctime);
        t = mktime(inctime);
        inctime = localtime(&t);

        strftime(date, 11, "%e.%m.%Y", inctime);
        printf("\ndate: %s\tmday= %d",date,int_mday);

        int_mday+=129;

        sprintf(str_mday, "%d",int_mday);
        strptime(str_mday, "%e", inctime);
        t = mktime(inctime);
        inctime = localtime(&t);

        strftime(date, 11, "%e.%m.%Y", inctime);
        printf("\ndate: %s\tmday= %d",date,int_mday);

        int_mday+=4129;

        sprintf(str_mday, "%d",int_mday);
        strptime(str_mday, "%e", inctime);
        t = mktime(inctime);
        inctime = localtime(&t);

        strftime(date, 11, "%e.%m.%Y", inctime);
        printf("\ndate: %s\tmday= %d\n\n",date,int_mday);


        return 0;
}



$ gcc -o mktime-test mktime-test.c && ./mktime-test

date: 24.03.2006        mday= 24
date:  5.03.2006        mday= 53
date: 18.03.2006        mday= 182
date:  4.03.2006        mday= 4311

------- Additional Comment #1 From Ulrich Drepper 2006-05-06 06:42 -------

This has nothing to do with mktime.  It is strptime which rejects your invalid
data.  This is the correct behavior.