cp -r /usr/share/grommunio-web/server/language/en_US.UTF-8/ usr/share/grommunio-web/server/language/en_GB.UTF-8/
Should copy the en_US locale settings that Grommunio uses to a real (! :) ) English locale of en_GB. Thereafter subject to having the en_GB locale installed in the OS then proper date formatting of dd/mm/yyyy will then be available in Gromunio instead of all that awkwardness of month/day/year that is so alien to most other English speakers!
Hope this helps!
Peter