For those fellow Brits out there I have a quick fix so that dates in email in the format dd/mm/yyyy and variants display the correct date in the popup
file name: /opt/zimbra/jetty-6.1.5/webapps/service/zimlet/com_zimbra_date/date.
js
In function ZmDate7ObjectHandler
Change
var month = parseInt(result[1], 10) - 1;
var dom = parseInt(result[2], 10);
To
var month = parseInt(result[2], 10) - 1;
var dom = parseInt(result[1], 10);
I just swapped the [1] and [2] around
You might also want to change ZmDate5ObjectHandler if you want to catch 25-1-2008 (dash format)
Restart zimbra for it to work