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

void print_date(const char *label, const struct tm *tm);
int date_equals(const struct tm *a, const struct tm *b);

int main(int argc, char **argv)
{
    time_t t_in = 0, t_ago;
    struct tm local, utc, local2;

    if (argc == 2) {
        t_in = (time_t) strtol(argv[1], NULL, 0);
    }
    local = *localtime(&t_in);
    print_date("Local", &local);
    utc = *gmtime(&t_in);
    print_date("UTC", &utc);

    /* subtract an interval of 34 years */
    local.tm_year -= 34;

    /* Normalize */
    local.tm_isdst = -1;
    t_ago = mktime(&local);
    if (t_ago == -1 && local.tm_isdst == -1) {
        int n = (70 - local.tm_year + 27) / 28;
        local.tm_year += n * 28;
        t_ago = mktime(&local);
        if (t_ago == -1 && local.tm_isdst == -1) {
            printf("Warning, time may be wrong\n"); /* or call elog() */
        }
        local.tm_year -= n * 28;
        t_ago -= (365*4+1)*7*24*60*60*n; /* assumes 1901 <= year <= 2099 */
    }

    print_date("Local", &local);
    local2 = *localtime(&t_ago);  /* this should be redundant */
    if (!date_equals(&local, &local2)) {
        print_date("ERROR", &local2);
    }
    utc = *gmtime(&t_ago);
    print_date("UTC", &utc);

    return EXIT_SUCCESS;
}

void print_date(const char *label, const struct tm *tm)
{
    char buffer[80];

    strftime(buffer, sizeof buffer, "%a %b %d %H:%M:%S %Y %Z %z", tm);
    printf("%5s %s\n", label, buffer);
    printf("      %4d-%02d-%02dT%02d:%02d:%02d, wday=%d, yday=%d, isdst = %d\n",
           1900 + tm->tm_year, 1 + tm->tm_mon, tm->tm_mday,
           tm->tm_hour, tm->tm_min, tm->tm_sec,
           tm->tm_wday, tm->tm_yday, tm->tm_isdst);
}

/* return 1 if all fields of a are equal to those of b, otherwise 0 */
int date_equals(const struct tm *a, const struct tm *b)
{
    return a->tm_year  == b->tm_year
        && a->tm_mon   == b->tm_mon
        && a->tm_mday  == b->tm_mday
        && a->tm_hour  == b->tm_hour
        && a->tm_min   == b->tm_min
        && a->tm_sec   == b->tm_sec
        && a->tm_wday  == b->tm_wday
        && a->tm_yday  == b->tm_yday
        && a->tm_isdst == b->tm_isdst;
}