This might be a no-brainer, but I thought it would be nice to mention it here anyways..

By default, when using English as your Zimbra webclient language, all time notations ( in your calendar or mails for example ) are in AM/PM notation. This can be quit annoying since us Europeans don’t use this system. 😉

The simple fix is to go to Preferences -> General -> Login Options -> Language, and select English ( United Kingdom ).

A simple reload later, and you’re still using your English webclient, but now with time in 24 hour format!