To cleanup !

Date Time format

Customize Date and Time formats in Thunderbird

Or this old info?

Date format trouble

NO Good:

Thunderbird Preferences > Advanced > General > Date and Time Formatting:

Regional settings locale: English (Denmark)

OK:

https://bugzilla.mozilla.org/show_bug.cgi?id=1426907#c59 

$ sudo ln -si /usr/share/i18n/locales/en_DK /usr/share/i18n/locales/root
 [sudo] password for mi:
$ echo 'root.UTF-8 UTF-8' | sudo tee -a /etc/locale.gen
 root.UTF-8 UTF-8
$ sudo locale-gen
 Generating locales (this might take a while)...
   de_CH.UTF-8... done
   en_AG.UTF-8... done
   en_AU.UTF-8... done
   en_BW.UTF-8... done
   en_CA.UTF-8... done
   en_DK.UTF-8... done
   en_GB.UTF-8... done
   en_HK.UTF-8... done
   en_IE.UTF-8... done
   en_IN.UTF-8... done
   en_NG.UTF-8... done
   en_NZ.UTF-8... done
   en_PH.UTF-8... done
   en_SG.UTF-8... done
   en_US.UTF-8... done
   en_ZA.UTF-8... done
   en_ZM.UTF-8... done
   en_ZW.UTF-8... done
   fr_CH.UTF-8... done
   root.UTF-8... done
 Generation complete.
$ env LC_TIME=root.UTF-8 thunderbird
 [calBackendLoader] Using Thunderbird's builtin libical backend
 *** UTM:SVC TimerManager:registerTimer called after profile-before-change notification. Ignoring timer registration for id: telemetry_modules_ping
$ sudo mcedit /usr/share/applications/thunderbird.desktop
$ grep TIME /usr/share/applications/thunderbird.desktop
 Exec=env LC_TIME=root.UTF-8 thunderbird %u

In TB: Help > Troubleshooting information :

Internationalisation & Localisation : Application Settings:

, , , ,