It's an old thread, but still appears in search.
So, easy solution found here:
Zimbra web: Language = English, Time format = 24 hour | Bart Verwilst a.k.a Bort :: Blog
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 ). This can also be performed on a CoS level for all users.
A simple reload later, and you’re still using your English webclient, but now with time in 24 hour format!
Please mark as Solved.