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

[Jonathan Wakely]

On Thu, 2 May 2024 at 19:35, Paul Eggert 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.

The patch looks great and resolves my concern, thanks!