Show glibc mktime unstable results when timezone changed on Europe/Lisbon and Europe/Istanbul. Europe/Amsterdam is an example where this unstable behaviour is not happening

test_europe_amsterdam_glibc.c

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

int main(int argc, const char * argv[]) {
    time_t ret, ret2, ret3;
    struct tm info, info2, info3;
    
    info.tm_year = 40;
    info.tm_mon = 5 - 1;
    info.tm_mday = 16;
    info.tm_hour = 0;
    info.tm_min = 0;
    info.tm_sec = 0;
    info.tm_isdst = -1;
    info.tm_wday = -1;
    info.tm_gmtoff = 0;
    info.tm_zone = 0;

    info2.tm_year = 40;
    info2.tm_mon = 1 - 1;
    info2.tm_mday = 1;
    info2.tm_hour = 0;
    info2.tm_min = 0;
    info2.tm_sec = 0;
    info2.tm_isdst = -1;
    info2.tm_wday = -1;
    info2.tm_gmtoff = 0;
    info2.tm_zone = 0;

    info3.tm_year = 40;
    info3.tm_mon = 5 - 1;
    info3.tm_mday = 16;
    info3.tm_hour = 0;
    info3.tm_min = 0;
    info3.tm_sec = 0;
    info3.tm_isdst = -1;
    info3.tm_wday = -1;
    info3.tm_gmtoff = 0;
    info3.tm_zone = 0;
    
    ret = mktime(&info);
    ret2 = mktime(&info2);
    ret3 = mktime(&info3);
    if( ret == -1 )
    {
        printf("Error: unable to make time using mktime\n");
    }
    else
    {
        char buff[70];
        printf("-------\n");
        printf("%s", asctime(&info));
        strftime(buff, sizeof buff, "%A %c %z", &info);
        puts(buff);
        printf("-------\n");
        printf("%s", asctime(&info2));
        strftime(buff, sizeof buff, "%A %c %z", &info2);
        puts(buff);
        printf("-------\n");
        printf("%s", asctime(&info3));
        strftime(buff, sizeof buff, "%A %c %z", &info3);
        puts(buff);
        printf("-------\n");
    }
    
    return(0);
}

test_europe_istanbul_glibc.c

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

int main(int argc, const char * argv[]) {
    time_t ret, ret2, ret3;
    struct tm info, info2, info3;
    
    info.tm_year = 10;
    info.tm_mon = 10 - 1;
    info.tm_mday = 1;
    info.tm_hour = 0;
    info.tm_min = 0;
    info.tm_sec = 0;
    info.tm_isdst = -1;
    info.tm_wday = -1;
    info.tm_gmtoff = 0;
    info.tm_zone = 0;

    info2.tm_year = 10;
    info2.tm_mon = 1 - 1;
    info2.tm_mday = 1;
    info2.tm_hour = 0;
    info2.tm_min = 0;
    info2.tm_sec = 0;
    info2.tm_isdst = -1;
    info2.tm_wday = -1;
    info2.tm_gmtoff = 0;
    info2.tm_zone = 0;

    info3.tm_year = 10;
    info3.tm_mon = 10 - 1;
    info3.tm_mday = 1;
    info3.tm_hour = 0;
    info3.tm_min = 0;
    info3.tm_sec = 0;
    info3.tm_isdst = -1;
    info3.tm_wday = -1;
    info3.tm_gmtoff = 0;
    info3.tm_zone = 0;
    
    ret = mktime(&info);
    ret2 = mktime(&info2);
    ret3 = mktime(&info3);
    if( ret == -1 )
    {
        printf("Error: unable to make time using mktime\n");
    }
    else
    {
        char buff[70];
        printf("-------\n");
        printf("%s", asctime(&info));
        strftime(buff, sizeof buff, "%A %c %z", &info);
        puts(buff);
        printf("-------\n");
        printf("%s", asctime(&info2));
        strftime(buff, sizeof buff, "%A %c %z", &info2);
        puts(buff);
        printf("-------\n");
        printf("%s", asctime(&info3));
        strftime(buff, sizeof buff, "%A %c %z", &info3);
        puts(buff);
        printf("-------\n");
    }
    
    return(0);
}

test_europe_lisbon_glibc.c

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

int main(int argc, const char * argv[]) {
    time_t ret, ret2, ret3;
    struct tm info, info2, info3;
    
    info.tm_year = 12;
    info.tm_mon = 1 - 1;
    info.tm_mday = 1;
    info.tm_hour = 0;
    info.tm_min = 0;
    info.tm_sec = 0;
    info.tm_isdst = -1;
    info.tm_wday = -1;
    info.tm_gmtoff = 0;
    info.tm_zone = 0;

    info2.tm_year = 11;
    info2.tm_mon = 1 - 1;
    info2.tm_mday = 1;
    info2.tm_hour = 0;
    info2.tm_min = 0;
    info2.tm_sec = 0;
    info2.tm_isdst = -1;
    info2.tm_wday = -1;
    info2.tm_gmtoff = 0;
    info2.tm_zone = 0;

    info3.tm_year = 12;
    info3.tm_mon = 1 - 1;
    info3.tm_mday = 1;
    info3.tm_hour = 0;
    info3.tm_min = 0;
    info3.tm_sec = 0;
    info3.tm_isdst = -1;
    info3.tm_wday = -1;
    info3.tm_gmtoff = 0;
    info3.tm_zone = 0;
    
    ret = mktime(&info);
    ret2 = mktime(&info2);
    ret3 = mktime(&info3);
    if( ret == -1 )
    {
        printf("Error: unable to make time using mktime\n");
    }
    else
    {
        char buff[70];
        printf("-------\n");
        printf("%s", asctime(&info));
        strftime(buff, sizeof buff, "%A %c %z", &info);
        puts(buff);
        printf("-------\n");
        printf("%s", asctime(&info2));
        strftime(buff, sizeof buff, "%A %c %z", &info2);
        puts(buff);
        printf("-------\n");
        printf("%s", asctime(&info3));
        strftime(buff, sizeof buff, "%A %c %z", &info3);
        puts(buff);
        printf("-------\n");
    }
    
    return(0);
}

to avoid this unstability problem, first init the tm structure with zeroes