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

[Guy Harris]

On Jan 6, 2024, at 12:52 PM, Paul Eggert <eggert at cs.ucla.edu> wrote:

> On 2024-01-06 12:25, Guy Harris via tz wrote:
> 
>> Not all users of the TZDB data use the tzdb code.  Some use the source
>> files rather than the TZif files, and I think some use the TZif files
>> but have their own code to read them
> 
> This partly depends on what one means by "their own code". On my Fedora 39 system, for example, even /usr/bin/zdump uses the GNU C Library's TZif-reading code, not tzcode's. Although there might be a distant relationship between glibc's TZif reader and tzcode's, by now they are quite separate code bases.
> 
> In contrast, the BSDs tend to use code derived from tzcode, though the code bases diverged years ago. Possibly these could be unified if someone had the time. Presumably that'd be a good thing....

I'm not referring to what the POSIX API implementations in Unix-like systems do, I'm referring to *other* users of the tzdb.  For example, ICU has its own code to read TZif files:

	https://github.com/unicode-org/icu/blob/main/icu4c/source/tools/tzcode/tz2icu.cpp