[tz] Why is "AEST" the abbreviation for Australia/Sydney in 1900?

[brian.inglis at systematicsw.ab.ca]

On 2024-05-03 04:14, Jonathan Wakely wrote:
> On Thu, 2 May 2024 at 23:25, brian.inglis--- via tz <tz at iana.org> wrote:
>>
>> On 2024-05-02 12:35, Paul Eggert via tz wrote:
>>> On 2024-04-30 16:21, Arthur David Olson via tz wrote:
>>>> For a given zone, each line describing the zone except for the last ends
>>>> with an until time. The creation of the transition time entry for the until
>>>> time is deferred until the following zone line has been completely
>>>> processed. (It happens at the bottom of the giant for loop in outzone.)
>>>> That deferral means that the time zone abbreviations in use have been
>>>> computed, so the appropriate abbreviation can be applied to the until time.
>>>
>>> OK, but this wouldn't address the issue of what happens with the last zone line,
>>> as that lacks an until time. For example:
>>>
>>>    Rule Aus 1917 only - Jan 1 2:00 1:00 D
>>>    Rule Aus 1917 only - Mar 1 2:00 0    S
>>>    Zone Australia/Sydney 10:00 Aus AE%sT
>>>
>>> zic generates a TZif file where time type 0 (the time type before 1917) uses the
>>> abbreviation "AEST" - but where did that "S" come from? The documentation
>>> doesn't say. I think this is Jonathan's main point.
>>>
>>> When computing %s for timestamps that come before the earliest rule, zic uses
>>> the LETTER/S field of the earliest rule that specifies standard time. I
>>> installed the attached to try to document this.
>>
>> I would expect it to default to "S" to allow "%s" in zones without rules.
> 
> Wouldn't that be wrong for several European countries using CE%sT
> where the LETTER/S for standard time is "-" and for daylight savings
> time is "S"?

There are no zones without rules in Europe data, and I never noticed that "S" is 
in alternate rather than standard time periods. In French Canada, it is HNE 
Normale rather than Hiver or HAE Avancée rather than Été.

As I said, the default is for zones without rules where you could assign 
letters, only standard time, about 1/3 of the total, depending on what you 
total, to allow "%s" in the abbreviation, which would have to default to "S".

Whereas in zones with rules you may have "D", sometimes "M" for Midsummer, and 
the whole abbreviation for UK and Russia: I wondered about MSK and in Cyrillic 
it appears to be MCK.
As the project and data is in English and not localized, that has to be handled 
by downstreams like ICU or apps.

-- 
Take care. Thanks, Brian Inglis              Calgary, Alberta, Canada

La perfection est atteinte                   Perfection is achieved
non pas lorsqu'il n'y a plus rien à ajouter  not when there is no more to add
mais lorsqu'il n'y a plus rien à retirer     but when there is no more to cut
                                 -- Antoine de Saint-Exupéry