The setting you mention is the one which is set up on the computer. Thunderbird uses whatever format your computer is set up to use. I can change the format to the same as what you see very easily.

• Exit Thunderbird

for Windows 10 OS

• In search type : date
• select: 'date & time settings'
• select: 'Region'

Under 'Regional format data', what do you see for the 'short date' format ?

• If you see 21/04/2020, click on 'Change data formats'
• For 'Short date', click on drop down and select 2020-04-21 format.

• Start Thunderbird.

The setting you mention is the one which is set up on the computer. Thunderbird uses whatever format your computer is set up to use.

Yes, that was the case with the old Thunderbird, but after upgrading to 68.4 not any more. At least, it looks like that to me.

for Windows 10 OS

I'm running Debian 10 OS, where you set the date format by invoking the command `sudo dpkg-reconfigure locales`. The `en_DK.UTF-8` locale is supposed to use ISO date format. With other programs, this works fine, e.g. with the `thunar` file browser, but not with Thunderbird, which seems to ignore the system settings.

Have you checked in preferences > advanced > General to see if it is set to use the system locale or the application locale.

It seems, that I'm out of luck. If I understand this bug report and its more than 160 comments over more than two years correctly, Thunderbird does support my use case on Windows, but not on Linux. It did work very well until version 58 or 59, but then the use of a new library broke the feature.

https://bugzilla.mozilla.org/show_bug.cgi?id=1426907

Developers seem to be aware of the problem now, but it will probably take some more years to solve it.

Thanks for your replies anyway!