But the message is shown (incorrectly) in the archive as having been posted on 26 September at 8:57 a.m. (out by +12 hours)
It should be shown as 25 September 8:57 p.m.
Where are you seeing this time? Is it the time displayed in the thread or message view or the time displayed as the Sender's time when you hover over the time in the thread or message view?
The incorrect date/time was showing in the archive thread view and also the archived message.
The server is set for UTC and the message displayed the date/time correctly in my desktop email (also UTC) - ie. the sender's header is 2025-09-25 20:57
I haven't knowingly made any changes to the Django settings (not sure how to check that - am yet to learn/understand the info in the link you've provided). The Mailman suite venv installation is "out of the box".
However, and I'm sort of embarrassed to say, I rebooted the server yesterday afternoon, and this morning the date/time in the archived message is displaying correctly (both thread and message).
I don't know if I missed my meds or the server just hiccupped - but I won't do anything unless it happens again.
Thank you Mark for replying, and apologies for taking up your time with what may yet turn out to be a false alarm.
Best, MarkD