[tz] [PROPOSED] Avoid backward links in zone.tab

[Paul Eggert]

On 5/10/21 3:43 AM, Michael H Deckers wrote:
> 
>     On 2021-05-10 08:38, Paul Eggert wrote:
>> The idea is to move Zones into 'backward' if they're identical to some 
>> other Zone after 1970.
> 
> 
>     If these rules really hold for backzone (are
>     they listed anywhere?),

theory.html says that tzdb records civil time by "partitioning the world 
into timezones whose clocks all agree about timestamps that occur after 
the POSIX Epoch". Although it's true that the existence of such a 
partition doesn't logically require that each Zone history be distinct 
after the Epoch, that is the intent nowadays. I suppose this should be 
stated more clearly once the patch is installed.
>        America/Curacao
>     also should go to backzone,

Yes, the proposed patch does this.

> and one of
>        Africa/Johannesburg and Africa/Maputo
>     and three of
>        Pacific/Majuro, Pacific/Tarawa,
>        Pacific/Funafuti, Pacific/Mata-Utu
>     should go to backzone.

Although Africa/Johannesburg and Africa/Maputo have identical UTC offset 
histories after 1970, they disagree about time zone abbreviations and so 
are not coalesced in the proposed patch.

The proposed patch does move those Pacific/* names to 'backzone', as 
they are all equivalent to Etc/GMT-12 after 1970. (tzdb uses 
'Pacific/Wallis' instead of 'Pacific/Mata-Utu'.)

>     By the way, do these rules imply that
>        Pacific/Yap
>     with the data as of 2005k belongs to backzone?

I don't recall that entry, but if someone would submit a patch (git 
format-patch format, please) for it we could add it to 'backzone'.

>     In previous versions, the name of a tzdb timezone in
>     backzone could also be found in zone.tab together with
>     its country code and the coordinates for its location
>     (except for backzone/Asia/Hanoi).

That was just luck having to do with a deprecated file; it wasn't 
intended. Perhaps someone could add systematically-formatted comments to 
'backzone' giving geographical coordinates for names like Asia/Hanoi 
that are not already mentioned in 'zone1970.tab'. I wouldn't rely on 
'zone.tab' as it's deprecated.

PS. All my 'backzone' suggestions in this email are low priority, as 
stuff moved to 'backzone' is out of scope for tzdb proper. That's the 
main point of 'backzone', after all - to lessen the burden of data 
outside the scope of the project.