The date format of the date column in Thunderbird/Seamonkey Mail/IceDove is based on your current locale. If you want to shorten in it in a meaningful way, you need to change the locale. This is not a good thing to do for just one program. Hence, put this command in a bash script and create a launcher that executes this script. The bash script changes the locale to Denmark (?) who apparently use the most sensible way of writing a datetime value.
export LC_TIME=en_DK.UTF-8 && seamonkey -mail
I guess if you use Thunderbird, you need to replace “seamonkey -mail” with “thunderbird” or “iceape”.