[tz] TZDB use cases

[Michael H Deckers]

    On 2021-10-04 01:45, Paul Eggert commented on my remark:
>> it is not even true that the two time scales
>>      always agree after 1970, although most people seem to believe
>>      otherwise.
>
> If there are counterexamples please suggest fixes, preferably in 'git 
> format-patch' form. Although 'backzone' is documented to be less 
> reliable, we're certainly open to fixes from the community.



    Before proposing premature fixes, I think it is appropriate
    to first agree and then specify what the backzone file should
         contain.

    The backzone file says:
         # This file contains data outside the normal scope of the tz 
database,
         # in that its zones do not differ from normal tz zones after 1970.
    If this were intended to mean that
         # The timezones in this file agree after 1970 with tz zones
         # in other files.
    then America/Ensenada must not occur in backzone. But the text does
    not say that, and America/Ensenada occurs in backzone; so we do
    not know what, if anything at all, is wrong here.

    The backzone file also says:
         # ... Many of
         # the zones were formerly in other source files, but were 
removed or
         # replaced by links as their data entries were questionable 
and/or they
         # differed from other zones only in pre-1970 timestamps.

    What is the purpose of keeping "questionable" versions of a timezone
    when we have a better version? If we know an error, it should be
    corrected -- but I do not see any purpose in keeping the incorrect
    version in backzone.
    And if we do not know where the errors are, then the timezone is in good
    company: except for a few carefully researched cases, we are never sure
    whether we have listed all transitions, or whether all transitions are
    correct. No need to keep it in backzone either.
    So the stuff about "questionable" data entries should disappear from
    backzone.

    Michael Deckers.