The theory, as you recognize, is that Thunderbird takes its date/time settings from the OS.

In Linux this is a bit more tricky to set up and stopped working for me. I gave in fighting it, and installed the Super Date Format add-on.

My only dissatisfaction with this add-on is that it doesn't appear to be able to display today's messages with just the time (as is Thunderbird's default), making them easier to spot in a listing.

Changing some settings via a GUI window can not, by any stretch of the imagination, be called "programmng", complicated or otherwise.

I downloaded the Super Date Format add-on, but not sure how to install it. The instructions say:

"How to Install in Thunderbird

1.   Download and save the file to your hard disk.
 2.  In Mozilla Thunderbird, open Add-ons from the Tools menu.
 3.   From the options button next to the add-on search field, select "Install Add-on From File..." and locate the downloaded add-on."

The problem for me is I don't see any "add-on search field", nor the adjacent "options button". So I've got the add-on, but can't figure out how to complete the install.

http://xenos-email-notes.simplesite.com/416814616

That button is now on the "Extensions" page, not the more logical "Gett add-ons" page.

But you should be able to use the search function to obtain the extension.

I've downloaded the Add-on, and I see it in the Downloads folder, but it doesn't appear in the Add-ons manager listing, so it can't install.

In Thunderbird go to menu Tools > Add-ons Manager, select the Extensions pane and type Super Date Format in the Search add-ons textbox in the top right and press Enter. Now you see the add-on and you can click Install to install it directly in Thunderbird. That's the easiest way to install an add-on.

Onno Ekker said

type Super Date Format in the Search add-ons textbox in the top right and press Enter

I did that, but I don't see a "Super Date Format" add-on. I only see a complicated text on IMAP formatting.