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, eg
zimbra.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://zimbra.opensure.net/dav/your@email.address/Calendar

or

https://zimbra.opensure.net/principals/users/your@email.address

Posted in: iPhone, OpenSure Guides, Zimbra Collaboration