Agreed, ideally we need to centralize that. And I already tried to do that by putting this all under single Change Language and Theme page to control for a user and to Webmin (Usermin) Configuration: Language and Locale page to set it globally for all users. This isn’t that bad.
But I almost completely forgotten that mail has its own date format options for some reason … Probably @Jamie could explain the reason behind it better, and/or remove those completely to rely only on whatever is defined on Webmin side (or Usermin side).
Could it be that I need to make a change at the OS level?
Not sure what you mean exactly … but the main obstacle is that we have two separate date formats (technically).
The old one is when the date generated on the server side, in the server executed scripts, which never reach/output to UI. In this case, it’s controlled by Server generated date format option in Locale configuration / Manual … the old location for this date control feature was Webmin (Usermin) Configuration: User Interface page, i.e. raw dateformat option. This old feature is not very flexible in providing locale based formatted date/time.
The new one, that happens on the theme side, which only receives the timestamp from the server, and process it in UI according to finesse user preferences. It’s much more flexible and powerful.
The best solution would be is to pre-process date/time solely on the server side. But that would require a lot of work (on Perl side) to support different formats and locales. This is not trivial. Actually, it’s like a separate project on its own.