That took me forever because I could not find the place where my Unity icons where to edit them in order to add the necessary LC_TIME definition. This being said, it's relatively easy except that you have no real control over the exact formt, only the locale.

So...

I created a file named starter under my thunderbird folder, if you don't have such (because you're using the stock version of thunderbird) then you may want to create a bin folder as in:

mkdir ~/bin

The starter file is just a text file with a shell script like this:

#!/bin/sh
LC_TIME=en_DK.utf8
export LC_TIME
unset LC_ALL
exec /home/alexis/tools/thunderbird/mozilla/dist/bin/thunderbird

The important part of the script is the LC_TIME=en_DK.utf8 entry. Change that definition to fit your need. To see a complete list of values you can use, type locale -a and then enter in your shell:

/home/alexis $ locale -a
C
C.UTF-8
en_AG
en_AG.utf8
en_AU.utf8
en_BW.utf8
en_CA.utf8
en_DK.utf8
en_GB.utf8
en_HK.utf8
en_IE.utf8
en_IN
en_IN.utf8
en_NG
en_NG.utf8
en_NZ.utf8
en_PH.utf8
en_SG.utf8
en_US.utf8
en_ZA.utf8
en_ZM
en_ZM.utf8
en_ZW.utf8
fr_BE.utf8
fr_CA.utf8
fr_CH.utf8
fr_FR.utf8
fr_LU.utf8
POSIX

Notice that most of these have .utf8 and if you don't include that it won't work.

Make sure that your script is executable:

chmod 775 ~/bin/starter

Now test by typing something like this:

/home/alexis/bin/starter

(obviously, put your home folder name instead of "alexis" unless you also use that name!?)

If that works, starts thunderbird and shows you the date you wanted to get, then you are ready to use that script to create a new launcher (or edit your existing one) and get your Thunderbird (or Shredder) to run exactly as expected each time. To edit launchers, you just right click on them and select Properties. If you don't see that option (i.e. you're in Ubuntu 11.10 or 12.04 or better) then you'll need to find the .desktop file. You may want to look at my New Ubuntu Desktop page (point 10).