Compute offset of local timezone from UTC for every day since 1880

GNUmakefile

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+CFLAGS = -std=c11 -Wall -ftrapv
+all: timezone-history

timezone-history.c

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+#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);
+    }
+}