[tz] Mongolia

[Heitor David Pinto]

Hello,
Sources about time zones in Mongolia seem to list one of two conflicting configurations. The first configuration, mentioned in a comment to the TZ database in 1999, citing a Mongolian government website, lists the provinces of Bayan-Ölgii, Khovd and Uvs in UTC+7, and the rest of the country in UTC+8. The second configuration, mentioned in a comment to the database in 2001, lists Bayan-Ölgii, Khovd, Uvs, Govi-Altai and Zavkhan in UTC+7, Dornod and Sükhbaatar in UTC+9, and the rest of the country in UTC+8.

The first configuration is still mentioned by several Mongolian travel agencies:
https://www.adventurerider.mn/en/page/about_mongolia 
http://www.naturetours.mn/nt/mongolia.php 
https://www.newjuulchin.mn/web/content/7506?unique=fa24a0f6e96e022a3578ee5195ac879638c734ce 

It also matches these flight schedules in 2013:
http://web.archive.org/web/20130722023600/https://www.hunnuair.com/en/timetabled 
The flight times imply that the airports of Uliastai (Zavkhan), Choibalsan (Dornod) and Altai (Govi-Altai) are in the same time zone as Ulaanbaatar, and Khovd is one hour behind.

It also matches a Mongolian news article about the election in 2017:
https://mongolia.gogo.mn/r/806d6 

The second configuration was mentioned by an official of the Mongolian standards agency in an interview in 2014:
https://ikon.mn/n/9v6 

And it's still listed by the Mongolian aviation agency:
 https://ais.mn/files/aip/eAIP/2023-12-25/html/eSUP/ZM-eSUP-23-04-en-MN.html 
The TZ database used the first configuration until 2001, when it switched to the second configuration, under the assumption that its sources were more reliable. In 2008, based on sources indicating that Choibalsan actually used UTC+8, the database changed Dornod and Sükhbaatar to UTC+8, but only from 2008 on, and maintained Govi-Altai and Zavkhan in UTC+7. This resulted in a mix of the two configurations that is not supported as a whole by any source.

Based on all the above sources, I believe that the first configuration is what is actually observed in Mongolia and has been so all along, at least since 1999. The second configuration closely matches the ideal time zone boundaries at 97.5°E and 112.5°E but it doesn't seem to be used in practice. So I suggest the following:
1. In zone.tab, remove Govi-Altai and Zavkhan from Asia/Ulaanbaatar, assuming that they have always used the same time as Ulaanbaatar, or create a new zone Asia/Uliastai showing Govi-Altai and Zavkhan in UTC+8 since at least 1999.
2. Remove the zone Asia/Choibalsan or make it an alias of Asia/Ulaanbaatar, assuming that it has always used the same time as Ulaanbaatar, or list its change from UTC+9 to UTC+8 since at least 1999 instead of 2008.
Regards,Dave

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