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

void test10()
{
    time_t t = 0;
    struct tm tm = {0};
    char buf[64];

    tm.tm_year = 2011 - 1900;
    tm.tm_mon = 12 - 1;
    tm.tm_mday = 31; // <-- here is the change
    tm.tm_hour = 0;
    tm.tm_min = 0;
    tm.tm_sec = 0;
    tm.tm_isdst = 1;

    strftime(buf, sizeof buf, "%F %T %Z", &tm);
    printf("before: %s %ld\n", buf, t);

    t = mktime(&tm);

    strftime(buf, sizeof buf, "%F %T %Z", &tm);
    printf("after1: %s %ld\n", buf, t);

    tm.tm_mday -= 1;
    t = mktime(&tm);

    strftime(buf, sizeof buf, "%F %T %Z", &tm);
    printf("after2: %s %ld\n", buf, t);

    tm.tm_mday += 1;
    t = mktime(&tm);

    strftime(buf, sizeof buf, "%F %T %Z", &tm);
    printf("after3: %s %ld\n", buf, t);
}

int main(int argc, char *argv[])
{
    test10();
    return 0;
}