[tz] Adapting localtime.c

[Robert Elz]

    Date:        Sat, 8 Oct 2016 21:59:44 +0100
    From:        John Haxby <john.haxby at oracle.com>
    Message-ID:  <E732F782-B470-4FF9-8754-CA018858AF94 at oracle.com>

  | The epoch is approximately=94 midnight at the beginning of 1970.

No, it is exactly the beginning of 1970 (UTC).   The issue is that the
"seconds" that are counted are not international standard seconds, but
posix seconds, which are defined to be 1/86400 of the length of the day
in which they occur.   That means that posix seconds can be slightly
(very slightly) longer (or in principal, shorter) than IS seconds.

If you're trying to measure precise time intervals, posix seconds are
useless (though they're fine if all you care about is the nearest 1/10
of a second or so) but for working with calendars, they're ideal, as there's
no need to keep a table of when leap seconds occur (which, as they're
unpredictable, is the only way to deal with them.)

kre