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

[Michael H Deckers]

    On 2024-01-05 23:31, Guy Harris via tz asked:

> The binary file obviously allows code that reads it to get information of the form "at date/time DT, one or more of {the offset from UTC, whether tm_isdst should be zero or non-zero, the time zone abbreviation} changes".  The tzcode doesn't happen to have APIs to*provide*  that information, but that's a different matter.
>
> Is there software that needs to know about transitions that change none of those?


> ...  So are there Java classes read those files and use them?
>
> Or are they files produced by Java code that*uses*  the data?


    Yes, there is information on local civil times scales needed by many
    datetime software interfaces that is already present in the tzdb 
source files
    but that is missing from the TZif files since the very beginning:

    • the SAVE value (or the numeric RULES value) applicable at an instant
      is not available via TZif files, and cannot in general be deduced
      from its contents (see eg the SAVE value +01 h for Europe/Dublin when
      UT = 1916-10-01 + 02:25:21.1). It has even become more difficult
      to guess these values from TZif files since they are allowed to be 
negative.

    • the RULEs applicable at a specific instant (if any). They are 
available
      in TZif files (in versions 2 and 3) only for recent instants, and 
the start
      of their applicability is only given indirectly in TZif files, 
sometimes
      requiring a redundant transition (which is the topic of this thread).

    I think it would be very useful to have an official output of the 
tzdb data
    compilation process (zic and associated tools) that makes these data
    available for datetime software, even though they are not needed by
    the POSIX datetime functions.

    Michael Deckers.