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

[Clive D.W. Feather]

Brooks Harris via tz said:
> For example, there is a STDOFF shift at 1971 Oct 31  2:00u from 1:00 to 0:00
> (a West shift) in London:
> 
> # Zone    NAME        STDOFF    RULES FORMAT    [UNTIL]
> Zone    Europe/London    -0:01:15 -    LMT    1847 Dec  1  0:00s
>              0:00    GB-Eire    %s    1968 Oct 27
>              1:00    -    BST    1971 Oct 31  2:00u
>              0:00    GB-Eire    %s    1996
>              0:00    EU    GMT/BST
> 
> Zic and TzIf reflect this change as a shift in gmtoff, not stdoff:
> 
>    57722399 1971-10-31 02:59:59 isdst 0 gmtoff   3600 stdoff      0 BST
>    57722400 1971-10-31 02:00:00 isdst 0 gmtoff      0 stdoff 0 GMT
> 
> That's what I mean by "adjusted" for Posix sake. It gives the proper UTC
> offset, yes, but not for the right reason. The underlying reason was an
> STDOFF shift, presumably stated in the law behind it.

The reason in this case was that the British Standard Time Act 1968 c.45
contained a sunset (sorry) clause:

4(2) Sections 1 to 3 of the Act shall expire at two o'clock, Greenwich Mean
Time, in the morning of 31st October 1971 unless made permanent under
subsection (3) below;

An attempt to make it permanent was defeated in the House of Commons on
1970-12-02 by 81 votes to 366. See Hansard HC Deb 02 December 1970 vol 807
cc1331-422. Therefore 4(2) came into effect and on that date the legal time
changed from that specified in the British Standard Time Act (GMT+1) to
that specified in the [1880 c.9 (43 & 44 Vict.).] Statutes (Definition of
Time) Act 1880 (GMT) as modified by the Summer Time Acts 1922 to 1947.

-- 
Clive D.W. Feather          | If you lie to the compiler,
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