#include <assert.h>
#include <stdint.h>
#include <stdio.h>
#include <time.h>

time_t start_time(void) {
    // return the start of our time range
    // we want 12:00 local time on January 1st of the first year >= 1880
    // we can represent (1880, because time zones based on GMT were
    // enacted in 1885, so we start a little bit earlier if we can)
    for (int y = 1880; y < 2000; y++) {
        struct tm tm;
        tm.tm_year = y - 1900;
        tm.tm_mon = 0;
        tm.tm_mday = 1;
        tm.tm_hour = 12;
        tm.tm_min = 0;
        tm.tm_sec = 0;
        tm.tm_isdst = -1;

        time_t t = mktime(&tm);
        if (t != (time_t)-1) {
            return t;
        }
    }
    assert(0); // no date before 2000 representable? mktime is broken
}

time_t end_time(void) {
    // return the end of our time range
    // 12:00 local time on dec. 31 of the current year seems ok.
    // we might want to extend that a few years into the future, but
    // it's hard to say how much: Sometimes DST changes are only
    // published weeks before they happen, so even the end of the year
    // is a bit optimistic.
    time_t t = time(NULL);
    struct tm *tmp = localtime(&t);

    struct tm tm;
    tm.tm_year = tmp->tm_year;
    tm.tm_mon = 11;
    tm.tm_mday = 31;
    tm.tm_hour = 12;
    tm.tm_min = 0;
    tm.tm_sec = 0;
    tm.tm_isdst = -1;

    t = mktime(&tm);
    if (t != (time_t)-1) {
        return t;
    }
    assert(0); // end of current year not representable? mktime is broken
}

int_least32_t diff(struct tm utm, struct tm ltm) {
    int_least32_t d;
    // local time should be close to noon, timezones are somwhere in
    // the +/- 14 hours range, so the UTC date can differ by at most one
    // date. Therefore we just check whether the UTC date is earlier or
    // later than the local date.
    if (ltm.tm_year > utm.tm_year) {
        d = 86400;
    } else if (ltm.tm_year < utm.tm_year) {
        d = -86400;
    } else {
        if (ltm.tm_mon > utm.tm_mon) {
            d = 86400;
        } else if (ltm.tm_mon < utm.tm_mon) {
            d = -86400;
        } else {
            if (ltm.tm_mday > utm.tm_mday) {
                d = 86400;
            } else if (ltm.tm_mday < utm.tm_mday) {
                d = -86400;
            } else {
                d = 0;
            }
        }
    }
    d += (ltm.tm_hour - utm.tm_hour) * 3600
       + (ltm.tm_min - utm.tm_min) * 60
       + (ltm.tm_sec - utm.tm_sec);
    return d;
}

int main(void) {
    time_t start = start_time();
    time_t end   = end_time();
    for (time_t t = start; t <= end; t += 24*3600) {
        struct tm utm = *(gmtime(&t));
        struct tm ltm = *(localtime(&t));
        int_least32_t d = diff(utm, ltm);
        printf("%.0f %.0f\n", (double)t, (double)d);
    }
}