[tz] OpenJDK/CLDR/ICU/Joda issues with Ireland change

[Robert Elz]

  From: Paul Eggert <eggert at cs.ucla.edu>
  Date: Sat, 27 Jan 2018 18:22:08 -0800
  Subject: Re: [tz] OpenJDK/CLDR/ICU/Joda issues with Ireland change

  | However, they do not suffice for tzdata, since they assume at
  | most two types of time can exist in a timezone_t object,

Actually, they don't, if zic set the isdst field in the ttinfo
to 0, 1, 2, 3, ... (allowed by POSIX< which just requires "positive"
"negative" and "zero") then the functions would fetch the corresponding
value (they don't really access tzname[] and timezone!)

  | So code should not use these functions if it wants to work on 
  | arbitrary timestamps.

But I agree with that, as there's no reason we cannot have isdst
value 1 with various different gmt offsets at different periods
(like when a zone has altered its base offset), and there's no
way with those functions to specify which one is wanted, nor
to doscover which was used for the result.

  | Also, even if one assumes POSIX, tzgetgmtoff and tzgetname
  | are not much use on platforms like NetBSD that have
  | tm_gmtoff and tm_zone.

They're less than "not much use", they are useless.

kre