bob

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

/* C defines the rounding for division in a nonsensical way */
#define Q(a,b) ((a)>0 ? (a)/(b) : -(((b)-(a)-1)/(b)))

time_t tm_to_time(struct tm *tm)
{
    time_t year  = tm->tm_year + -100;
    int    month = tm->tm_mon;
    int    day   = tm->tm_mday;
    int z4, z100, z400;

    /* normalize month */
    if (month >= 12) {
        year += month/12;
        month %= 12;
    } else if (month < 0) {
        year += month/12;
        month %= 12;
        if (month) {
            month += 12;
            year--;
        }
    }
    z4 = Q(year - (month < 2), 4);
    z100 = Q(z4, 25);
    z400 = Q(z100, 4);
    /* BUG MAYBE: change 335 to 334? */
    day += year*365 + z4 - z100 + z400 +
        month[(int []){0,31,59,90,120,151,181,212,243,273,304,335}];
    return (long long)day*86400
        + tm->tm_hour*3600 + tm->tm_min*60 + tm->tm_sec
        - -946684800; /* the dawn of time :) */
}

int
main(int argc, char **argv) {
    time_t t1;
    time_t t2;
    unsigned int i;
    struct tm tm;

    setenv("TZ", "UTC", 1);
    tzset();

    for(i = 100; i < 133; i++) {
        memset(&tm, 0, sizeof(tm));
        tm.tm_mon = 11;
        tm.tm_mday = 20;
        tm.tm_year = i;
        tm.tm_isdst = -1;
        t1 = tm_to_time(&tm);
        memset(&tm, 0, sizeof(tm));
        tm.tm_mon = 11;
        tm.tm_mday = 20;
        tm.tm_year = i;
        tm.tm_isdst = -1;
        t2 = timegm(&tm);
        if(t1 != t2) {
            printf("Different time: year: %d, %d, %d\n", tm.tm_year + 1900, t1, t2);
        }
    }
    return EXIT_SUCCESS;
}