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

[Paul Eggert]

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'

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.