TimezoneConverter has documentation and is on Packagist
I made a page for a new extension: https://www.mediawiki.org/wiki/Extension:TimezoneConverter
It (and the extension) still need lots of work. It seems there are issues on some devices with timezone abbreviations (e.g. ‘AWST’ gets shown as ‘GMT+8’ on my Android). I don’t want to switch to Moment.js, but maybe that’s the easiest complete database.