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

[Paul Eggert]

On 5/9/21 3:41 AM, Michael H Deckers wrote:

>>         +VN    +1045+10640    Asia/Bangkok    Vietnam (north)
> 
> 
>      Asia/Hanoi was not added to backward/ as a link, so it
>      seems to be inappropriate to add it to zone.tab.

Asia/Hanoi is different from most of the other cases, as the situation 
with Hanoi was discovered after the guidelines were changed (so there 
was no need to add Asia/Hanoi) whereas most of the other cases (e.g., 
America/St_Thomas) were added before that. For example, had the new 
guideline been in place earlier, zone.tab would still have its current 
line "VI +1821-06456 America/Port_of_Spain" even though there would be 
no America/St_Thomas link in 'backward'.

By the way, that "VI +1821-06456 America/Port_of_Spain" is not right; 
that line should refer to America/Puerto_Rico instead. Port-of-Spain and 
San Juan, Puerto Rico have agreed since 1970 and the latter is bigger, 
so America/Port_of_Spain should also be moved to 'backzone'. There are 
several other examples like that, and we could shrink the installed set 
of TZif files significantly if this sort of thing were fixed. I plan to 
take a look into that.

>      But if it is added, then the coordinates should not
>      be those of Ho Chi Minh City but those of Hanoi:
>                +2102+10551

Thanks, good point, I installed the attached. Although zone.tab is 
deprecated I imagine some apps are still using it.
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