[tz] Europe/Tiraspol problem

[Benjamin Drung]

Hi Yoshito,

On Fri, 2023-05-19 at 21:20 +0000, Yoshito Umaoka via tz wrote:
> Dear all,
>  
> I was looking at some Unicode ICU test failure on Ubuntu related to
> time zone.
>  
> ICU utilizes TZ database including backward file. In backward file,
> Europe/Tiraspol is defined as a link of Europe/Chisinau. So the actual
> rule is coming from Europe/Chisinau as below (2023c)
>  
> # Zone   NAME                   STDOFF RULES    FORMAT               [UNTIL]
> Zone      Europe/Chisinau               1:55:20 -              LMT       1880
>                                                 1:55       -               CMT       1918 Feb 15 # Chisinau MT
>                                                 1:44:24 -               BMT       1931 Jul 24 # Bucharest MT
>                                                 2:00       Romania               EE%sT   1940 Aug 15
>                                                 2:00       1:00       EEST       1941 Jul 17
>                                                 1:00       C-Eur     CE%sT   1944 Aug 24
>                                                 3:00       Russia    MSK/MSD            1990 May  6  2:00
>                                                 2:00       Russia    EE%sT   1992
>                                                 2:00       E-Eur      EE%sT   1997
> # See Romania commentary for the guessed 1997 transition to EU rules.
>                                                 2:00       Moldova               EE%sT
>  
> I found behavior of Europe/Tiraspol is different on Ubuntu. When I set
> Europe/Tiraspol as the system time zone, the default offset set to
> timezone variable becomes -10800 (3 hour ahead of UTC). However,
> Europe/Chisinau above shows it should be -7200 (2 hour ahead of UTC)
> at this moment. (POSIX timezone variable should store “standard
> offset”, not counting daylight adjustment).
>  
> On Ubuntu, when I set Europe/Tiraspol as system time zone, -10800 is
> set to timezone variable. However, when I set Europe/Chisinau, it
> changes to -7200. Then I realized Ubuntu is using backzone file, which
> contains different rule.
>  
> In backzone file, Europe/Tiraspol is defined as below (2023c)
>  
> # Moldova / Transnistria
> Zone      Europe/Tiraspol 1:58:32 -               LMT       1880
>                                                 1:55       -               CMT       1918 Feb 15 # Chisinau MT
>                                                 1:44:24 -               BMT       1931 Jul 24 # Bucharest MT
>                                                 2:00       Romania               EE%sT   1940 Aug 15
>                                                 2:00       1:00       EEST       1941 Jul 17
>                                                 1:00       C-Eur     CE%sT   1944 Aug 24
>                                                 3:00       Russia    MSK/MSD            1991 Mar 31  2:00
>                                                 2:00       Russia    EE%sT   1992 Jan 19  2:00
>                                                 3:00       Russia    MSK/MSD
>  
> I understand backzone was created for preserving zone definition of
> some locations which only differs pre-1970 date, and these data might
> not be reliable. In reality, I think most of TZ database consumer
> still rely on either backward or backzone for backward compatibility.
> I think it’s problematic if these two files produce different results
> today.
>  
> I guess the zone definition in backzone file might be outdated, and
> Tiraspol is actually UTC+2/+3.

We (or better only blame me) enabled using backzone in Ubuntu recently
(see https://launchpad.net/bugs/2003797), but noticed that this changes
Europe/Tiraspol and some other timezones for post-1970 dates
(see https://launchpad.net/bugs/2017999). So we are building tzdata with
PACKRATLIST=zone.tab now to fix that. Please follow
https://launchpad.net/bugs/2017999 for details and progress.

-- 
Benjamin Drung
Debian & Ubuntu Developer