[tz] Extra transition for Europe/London with 2023d

[Arthur David Olson]

> ...when tm_gmtoff was introduced, there was no tm_gmtoff in POSIX or, for
that matter, in most UN*Xes (was it in *any* UN*Xes before tzcode put it
in?)...

Some tm_gmtoff history. One general note: the earliest time zone package
work was influenced by what had been done for UNIX System V, BSD, and HP-UX.

A source for a UNIX System V manual:

http://bitsavers.trailing-edge.com/pdf/att/3b1/999-801-312IS_ATT_UNIX_PC_System_V_Users_Manual_Volume_1.pdf
System V version 3.51 (1986) did not have tm_gmtoff.

A source for BSD-derived manuals:
    https://man.freebsd.org/cgi/man.cgi/help.html
Neither HP-UX 8.07 (1991) nor HP-UX 11.22 (2002) had tm_gmtoff, so it would
not have been present before those dates.

The earliest 1986 version of the time zone package (distributed on request)
did not have tm_gmtoff.

The 1987-02-28 version of the time zone package distributed via USENET's
mod.sources did have tm_gmtoff.
    https://groups.google.com/g/mod.sources/c/2Jq1irYs0w4
tm_gmtoff and tm_zone were added at the same time; there's material in the
IANA mailing list archive in late 1986 and early 1987 on making additions
to struct tm.

And a month after the mod.sources posting...
    From ado Sat Mar 28 15:13:19 1987
    To: tz
    Subject: Berkeley variant of time zone stuff
    Status: RO
    ...
    Berkeley forged ahead and added the "tm_gmtoff" and "tm_zone" elements
to
    the tm structure.
    ...
...meaning that it wasn't present in BSD before then.

    @dashdashdo

On Sat, Jan 6, 2024 at 7:07 PM Guy Harris via tz <tz at iana.org> wrote:

> On Jan 6, 2024, at 3:35 PM, Paul Eggert <eggert at cs.ucla.edu> wrote:
>
> > On 2024-01-06 14:51, Guy Harris via tz wrote:
> >
> >> POSIX's time API has no notion of "gmtoff" and "stdoff".
> >
> > Although that's true for current POSIX, POSIX 202x/D3 does have
> tm_gmtoff in struct tm, as a result of Austin Group Defect 1533 <
> https://austingroupbugs.net/view.php?id=1533> which saw steffen and kre
> as contributors.
>
> Yes, I know, I signed up for the list so that I could read the draft.
>
> But there's no tm_stdoff, so it wasn't obvious that "gmtoff" and "stdoff"
> were referring to values in struct tm, and Brooks Harris said
>
> > I just meant the values of gmtoff and stdoff are adjusted to satisfy
> Posix-time rather than reflect the values in the source files.
>
> but tm_gmtoff is currently obviously not "adjusted to satisfy Posix-time"
> as, when tm_gmtoff was introduced, there was no tm_gmtoff in POSIX or, for
> that matter, in most UN*Xes (was it in *any* UN*Xes before tzcode put it
> in?), and it's also not "adjusted", it's just the current offset as
> calculated from the values in the source files.
>
> And, as there's no "tm_stdoff" value even now, there's nothing *to* adjust
> about stdoff.
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