[tz] [PROPOSED] Merge timezones that are alike since 1970

[Paul Eggert]

On 5/30/21 8:10 AM, David Patte via tz wrote:

> 1) On one hand, several people would like to simplify the main data 
> table, reducing the number of individual zone records by merging them 
> where they are the same since 1970....
> 
> 2) On the other hand, several people would like to have tz be a source, 
> maintain and collect more historical tz data and organize it by 
> international standards so the historical data can be expanded per 
> country....
> 
> So, I have a question. And I'm not trying to difficult.
> 
> Assuming that the goal of (1) is achieved, what can be done to not 
> undermine type 2 users? If we have (1), but also want to maintain and 
> expand historical data per country's zones, how could that be done? 
> using a secondary table? Could the back table be improved, expanded and 
> enhanced?

These are good questions. We have the 'backzone' file for type (2) data, 
but the 'backzone' approach is apparently unsatisfactory for some users 
of type (2) data.

Part of the problem may be that 'backzone' is a mishmash quality-wise. 
Today there are only two simple options in the Makefile - use either all 
of 'backzone' or none of it - and there are real problems with using all 
of it. Perhaps we could add something that would give users more 
flexibility.

For example, if a downstream user wants the 'backzone' entry for 
Europe/Stockholm which is well-documented, but doesn't want backzone's 
America/Montreal entry because it's not well-attested and is most likely 
wrong, the user could specify a list of backzone names that includes 
Europe/Stockholm but excludes America/Montreal. I think it would not be 
too much work to add something like this to the tzdb code. This would 
better support users who want a separate Zone for each country in 
'backzone', but do not want all of 'backzone'.