By Default Qubes (4.01) uses the C.UTF-8 locale, resulting in Thunderbird formatting dates as MM/DD/YYYY.
To utilise the more sensible date formatting of ISO-8601 and format dates as DD/MM/YYY, create the following files and restart the Qube once complete.
/rw/config/locale.conf
LANG=en_GB.UTF-8
/rw/config/rc.local
rm -rf /etc/locale.conf
ln -s /rw/config/locale.conf /etc/locale.conf
localectl