EFUNs [AIX,NT]: Improved support for time beyond 2038-01-19 in mktime().

Fixes calculation for dates past 2038-01-19T03:14:07 UTC.

The issue was caused by the C division operator rounding negative
values toward zero rather than toward negative infinity. Now the
dividend should stay positive to avoid surprises.

Note that mktime() is best-effort for timezone offsets other than 0.

src/builtin_functions.c

@@ -5994,11 +5994,17 @@ time_t mktime_zone(struct tm *date, int other_timezone, int tz)
    * The calendar repeats every 28 years, so offset it appropriately.
    * Offset years before 1971 in order to avoid issues near 1970-01-01.
    */
-  if ((date->tm_year <= -300) || (date->tm_year > 130)) {
+  if (date->tm_year <= -300) {
     int y = (100 - date->tm_year)/400;
     date->tm_year += y * 400;
     ydelta += y * 400;
     retval -= y * 12622780800LL;	/* 400 years in seconds. */
+  } else if (date->tm_year > 130) {
+    /* NB: Duplicate of the above due to rounding to zero. */
+    int y = (date->tm_year + 300)/400;
+    date->tm_year -= y * 400;
+    ydelta -= y * 400;
+    retval += y * 12622780800LL;	/* 400 years in seconds. */
   }
   if (date->tm_year <= -30) {		/* Before 1870. */
     int y = -date->tm_year/100;