Reason
Unmaintained for few years, have few unresolved task, maintainer is not active too. Doesn't appear to have any WikiApiary page (or any mentions on WikiApiary at all), so it's either unused or nearly so.

To-do list

• This task
  • Add maintainers of that extension/skin as subscribers to this task (if they have no apparent Phabricator account, notify them on-wiki or via email if possible and note it here).
  • Add Phabricator project <#ProjectX> as a tag for this task, and remove MediaWiki-extensions-Other as appropriate.
• On-wiki documentation
  • Archive documentation on mediawiki.org (https://www.mediawiki.org/wiki/Extension:TimezoneSelector): replace page contents with {{Archived extension|3504484|reason=archive reasons, per {{phab|T237894}}.}}.
  • If documentation page was translatable, remove <translate>, visit Special:PageTranslation, and click "remove from translation" (if you don't have the translation administrator right, ask a user who does).
  • Update Wikidata item (https://www.wikidata.org/wiki/Q21678877) associated with documentation page
    • add statement Abandonware (Q281039) to instance of (P31) together with qualifier start time (P580) = the YYYY-MM-DD date that you decided to archive extension (generally per edit history)
    • add qualifier end time (P582) = the YYYY-MM-DD (same date as above) to `instance of (P31) = MediaWiki extension (Q6805426)
• Phabricator
  • Mark all Phabricator tasks for the extension either Declined or Invalid.
  • Archive Phabricator project <#ProjectX> for the extension.
  • Edit Phabricator project <#ProjectX> description for the extension with a link to this ticket.
• Translatewiki.net/translations
  • Remove from translatewiki.net ([[ https://gerrit.wikimedia.org/r/plugins/gitiles/translatewiki/+/refs/heads/master/groups/MediaWiki/mediawiki-extensions.txt | groups/MediaWiki/mediawiki-extensions.txt ]]).
• Configuration/tests/integrations/etc.
  • Remove from [[ https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/extensions/+/master | mediawiki/extensions ]] (along with in [[ https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/extensions/+/master/.gitmodules | .gitmodules ]]).
  • Remove tests in [[ https://gerrit.wikimedia.org/g/integration/config | integration/config ]] ([[ https://gerrit.wikimedia.org/r/plugins/gitiles/integration/config/+/master/zuul/layout.yaml | zuul/layout.yaml ]]).
  • Remove manifest/other references from [[ https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/vagrant/+/master | mediawiki/vagrant ]] if they exist.
  • Remove from https://doc.wikimedia.org/cover-extensions/ if present (requires Continuous-Integration-Infrastructure shell user to delete directly from the server).
• Repositories
  • Empty repository, leaving only the README indicating the repo is archived, and referring to this task by URL.
  • Mark the repository read-only in Gerrit (Edit description to prepend "[ARCHIVED]" and append Task reference): https://gerrit.wikimedia.org/r/#/admin/projects/mediawiki/extensions/TimezoneSelector
  • Archive the diffusion repository for the extension: rETZS
  • Archive the GitHub mirror: https://github.com/wikimedia/mediawiki-extensions-TimezoneSelector