[tz] Help for TZDB localization downstream

[Paul Eggert]

[Retitling from "Single source of truth for timezone data" as per 
<https://data.iana.org/time-zones/theory.html#accuracy>.]

On 2022-12-02 22:57, Guy Harris via tz wrote:
> So we could, in principle:
> 
> 	have new versions of the source files with an additional METAZONE field at the end of Zone lines;
> 
> 	have zic handle that field;
> 
> 	have a tool to strip off the METAZONE field, to produce files to be handled by tools that read source files and that*don't*  understand the METAZONE field;

Good suggestion. Possibly even better would be leave the current files 
alone (this would cause less disruption downstream), and have a new file 
containing a table that maps abbreviations+contexts to longer names that 
disambiguate the abbreviations. These longer names could then be used by 
CLDR and any other localization efforts. Something like this, say:

    ACST * Australian Central Standard
    ACDT * Australian Central Daylight
    AST  * Atlantic Standard
    ADT  * Atlantic Daylight
    APT  * Atlantic Peace
    AWT  * Atlantic War
    ADDT * Atlantic Double Daylight
    ...
    CST America/* Central Standard
    CST Asia/*    China Standard
    ...
    IST Asia/Jerusalem Israel Standard
    IST Asia/Kolkata   India Standard
    IST Europe/Dublin  Irish Standard
    ...
    BMT Asia/Jakarta     Batavia Mean
    BMT Europe/Zurich    Bern Mean
    BMT America/Barbados Bridgetown Mean
    BMT Europe/Chisinau  Bucharest Mean

where the xMT abbreviation list is shortened by omitting entries like 
the following as they can be deduced automatically:

    BMT Asia/Baghdad     Baghdad Mean
    BMT Asia/Bangkok     Bangkok Mean
    BMT America/Bermuda  Bermuda Mean
    BMT America/Bogota   Bogota Mean
    BMT Europe/Brussels  Brussels Mean
    BMT Europe/Bucharest Bucharest Mean

and with the convention that LMT always denotes local mean time when it 
appears in a Zone line (as opposed to a continuation line).

This new table would be a machine-processable improvement on the 
less-formal abbreviation lists that are already published here:

https://data.iana.org/time-zones/theory.html#abbreviations

Such a table should be relatively easy to maintain, as the set of 
abbreviations is small and rarely changes now that we've removed 
invented abbreviations. Of course it would require some work from the 
CLDR side, if they want to use this new table instead of doing things 
their current way, so it'd be nice to hear from the CLDR side before 
proceeding.