You can use the open standard called CalDav to synchronise your Zimbra calendar to your iPhone or iPad.
You are connecting to your Zimbra account via a special URL over which Zimbra allows connection to your calendar
To configure this on your iPhone or iPad open iPhone Settings application and choose Mail, Contacts, Calendar:
Then select Add Account…:
press Other:
Choose Add CalDAV Account:
Enter your Zimbra server hostname in the Server field,
mail.opensure.net
provide your username and password then press Next:
Username = your@email.address
password = password
On the account page, open Advanced Settings:
Enable SSL and check port, make sure Account URL resembles:
https://mail.opensure.net/dav/your@email.address/Calendar
or
https://mail.opensure.net/principals/users/your@email.address
Posted in: iPhone, OpenSure Guides, Zimbra Collaboration