mktime() failure vs 31-DEC-1969 23:59:59 GMT

[Roland McGrath]

The standard way in ANSI C to detect errors when the return value is
ambiguous is using `errno':

{
  errno = 0;
  timet = mktime (&stm);
  if (timet == (time_t) -1 && errno != 0)
    perror ("mktime");
}