Krdytkyu

Ok, solved it:

1) Make sure you have the locale you need, can't say which you specifically need but when you know you create it like this (using en_DK.utf8)

sudo locale-gen en_DK.utf8

2) To make sure this locale is in effect for thunderbird you add it to the script that starts thunderbird, so first find that script:

2a) find the right script

which thunderbird

In my case: /usr/bin/thunderbird

2b) add locale to the script (I use the editor geany):

gksudo geany /usr/bin/thunderbird

Add this in the beginning of the script (I just put at the very beginning):

LC_ALL="en_DK.utf8"

export LC_ALL

Just want to add this:

https://help.ubuntu.com/community/Locale

EDIT: as pointed out by pl1nk A better solution would be to NOT touch the /usr/bin/thunderbird script and instead create the script '/usr/local/bin/thunderbird' with this content

#!/bin/sh

LC_ALL="en_DK.utf8"

export LC_ALL

/usr/bin/thunderbird $@

make sure it's executable

sudo chmod a+x /usr/local/bin/thunderbird

Then check if it's being used to start thunderbird:

which thunderbird

should respond with this:

/usr/local/bin/thunderbird

Now thunderbird can be started as before.

There's a Super Date Format thunderbird addon:

LC_TIME=en_DK.utf8 thunderbird

I run my system as en_US.UTF-8 too, just in case....

Thunderbird 60

The way dates and times are formatted in Thunderbird 60 has changed. The following will provide a date/time format that will look like this: 2018-12-04 14:23:

1. 
   

   Create the root locale

   
   
   

   sudo ln -s /usr/share/i18n/locales/en_DK /usr/share/i18n/locales/root
   
   sudo sh -c "echo 'root.UTF-8 UTF-8' > /var/lib/locales/supported.d/local"
   
   sudo locale-gen
   
   

   
   


2. 
   

   Copy the Thunderbird launcher locally

   
   
   

   cp /usr/share/applications/thunderbird.desktop ~/.local/share/applications/
   
   

   
   


3. 
   

   Change the date/time locale for Thunderbird

   
   
   

   sed -i.bak 's/^Exec=thunderbird %u/Exec=env LC_TIME=root.utf8 thunderbird %u/' ~/.local/share/applications/thunderbird.desktop
   
   

   
   

Thunderbird 59 and below

Fsando's answer works, but LC_ALL will change the entire locale (date, number, currency format, etc) used by Thunderbird instead of just the date/time format, which is all that's asked for in the question. Not only that, but I don't like creating extra scripts if I don't have to. Here's what I did:

1. 
   

   Make sure the en_DK.utf8 locale is available (it should already be available if your desktop language is English):

   
   
   

   locale -a | grep en_DK
   
   

   
   


2. 
   

   If it's not, install the locale, the official way:

   
   
   

   sudo apt-get -y install language-pack-en
   
   

   
   
   

   Or if you don't feel like installing extra packages:

   
   
   

   sudo locale-gen en_DK.utf8
   
   

   
   


3. 
   

   Copy the Thunderbird launcher locally

   
   
   

   cp /usr/share/applications/thunderbird.desktop ~/.local/share/applications/
   
   

   
   


4. 
   

   Change just the date/time locale for Thunderbird

   
   
   

   sed -i.bak 's/^Exec=thunderbird %u/Exec=env LC_TIME=en_DK.utf8 thunderbird %u/' ~/.local/share/applications/thunderbird.desktop
   
   

   
   



5. If you're using Xfce the change is picked up right away, but if you're using Unity you may have to log out/log back in. Not sure about GNOME.

Next time you open Thunderbird from your launcher, it should use the new date/time format.

Advantages:

• Only overrides the date/time format


• No extra scripts necessary


• Only makes the change for your user, not all users on the system

And as a bonus, the change shouldn't get overwritten when the thunderbird package gets updated, because it won't touch your local launcher file.

Source:
http://kb.mozillazine.org/Date_display_format

Note: As Sparhawk mentions, LC_TIME will change date format as well as time format. However, you can find a locale with the same date format and different time format, and thereby change only the time format.

For example, this is what the en_US.utf8 locale looks like:

$ python3 -c "import locale, time; locale.setlocale(locale.LC_TIME, 'en_US.utf8'); print(time.strftime('%x %X'))"

12/05/2018 03:40:50 PM

Changing the locale to en_DK.utf8 will change the date format too:

$ python3 -c "import locale, time; locale.setlocale(locale.LC_TIME, 'en_DK.utf8'); print(time.strftime('%x %X'))"

2018-12-05 15:41:14

I just added LC_TIME=en_DK.UTF-8 to /etc/default/locale. Works fine on Linux Mint 17.3, should work in Ubuntu too.

1) open /etc/default/locale in your editor. The content of the file should look something like this:

LANG="en_US.UTF-8"

LC_NUMERIC="en_US.UTF-8"

LC_MONETARY="en_US.UTF-8"

LC_PAPER="en_US.UTF-8"

LC_NAME="en_US.UTF-8"

LC_ADDRESS="en_US.UTF-8"

LC_TELEPHONE="en_US.UTF-8"

LC_MEASUREMENT="en_US.UTF-8"

LC_IDENTIFICATION="en_US.UTF-8"

2) add LC_TIME=en_DK.UTF-8. If LC_TIME= is already there, change its value to en_DK.UTF-8.

3) Save and restart OS.

/etc/default/locale is not thunderbird specific. If you change the format there it will probably apply to other applications as well.