Actually, this is what worked for me hopefully this allows you to do it too.
1. Go into the Zimbra calendar tab and right click on the calendar
you would like to share and click on "Share Calendar". Copy the URL at
the bottom of the pop up window.
2. Go into your Google
Calendar under "Settings" --> "Calendars" --> "Import Calendar"
--> "Add by URL" and right click and paste the URL you copied from
Zimbra.
3. Fix the URL so that it goes from this
https://castor.rkk.com:443/home/user...@rkk.com/Calendar to this
https://castor.rkk.com/home/username/Calendar
4. Click "Add" and the calendar from Zimbra should show up in your google calendar.
(of
course this is using our server name (?) "
castor.rkk.com") I'm not sure if this is what you had tried already but hopefully it works! Let me know if this works for you, I have sent this to other people in my company and it worked for them but I don't know if it will for anyone else outside of that.
L.