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

[Brooks Harris]

On 1/6/2024 3:31 PM, Paul Eggert wrote:
> On 2024-01-06 09:55, Brooks Harris via tz wrote:
>> in many situations, such as STDOFF shifts, STDOFF shifts simultaneous 
>> with DST shifts, or "double summertime" the TzIf values are 
>> essentially 'lying'
Sorry. "Lying" might be the wrong word, didn't mean to insult TzDb. I 
just meant the values of gmtoff and stdoff are adjusted to satisfy 
Posix-time rather than reflect the values in the source files.
>
> Surely this goes too far. Whether a particular UT offset is "standard 
> time" or "daylight saving time" (or something else) does not concern 
> ordinary users and is pretty much arbitrary - as witness the 
> disagreement over whether Morocco observes "standard time" or 
> "daylight saving time" for most of the year. To this end, the TZif 
> values are not lying; they're merely limiting themselves to the info 
> needed to display timestamps.
>
> What matters to users is "What time is it?". Questions like "Is 
> daylight saving time observed now?", "Is daylight saving ever 
> observed?" and "What is the standard time now, ignoring any DST 
> observance?" are timekeeping nerds' means to that end, not the end 
> itself, and are best left to tzcode's internals.
>
> This is why theory.html says "The tm_isdst member is almost never 
> needed and most of its uses should be discouraged....". In hindsight 
> there never should have been a tm_isdst (instead, there should have 
> been tm_gmtoff and tm_zone) though obviously it's too late now to 
> remove tm_isdst.