The OpenStreetMap conference, "State of the Map", has an impressive, easy to use, tool for switching timezones on its programme page: https://2021.stateofthemap.org/programme/
Could we use that, or something very like it, on Wikipedia and sister projects?
In theory, yes. It looks like they're using https://momentjs.com/timezone/, which is MIT licensed (so suitable for use here too), then using a fairly simple JQuery-based script to change the timezones. It looks like moment.js is already available in ResourceLoader, but the moment-timezones library isn't. It's also worth noting that this is fairly similar to what w:Wikipedia:Comments in Local Time and w:User:Mxn/CommentsInLocalTime do already (though without a timezones library, as they get the local timezone from the browser). This talk page isn't heavily watched, so you'd probably find someone more able to help elsewhere.
Thanks: I tried asking the above on the "Village Pump (Technical)" on English Wikipedia and my query was archived unanswered. Where else do you suggest I try?