1. export your Zimbra calendar in iCal format on your Zimbra server
2. use the Zimbra Apache server to publish the iCal files on the Internet
3. create a new Calendar in Google Calendar and subscribe to a remote URL (bummer that Google cannot do URL authentication)
4. Google will then automatically retrieve this public URL, consume the iCal file and update the calendar. This happens about every 12 to 24 hrs, depending on same random Google'ness.

1. .ics files are available on the Internet without any password protection, it's only security by obscurity. But the URL is pretty impossible to guess as it must be the exact .ics file name
2. Google will update the calendar every 12 to 24 hrs

To set this up:

1. The Zimbra Apache web server is only accessible on Port 7780 which is not open by default to the Internet. So open a random port on your Firewall (some random high port) and redirect that to Port 7780 (for TCP only). You can also lock this down to only permit access from Google servers, if you can figure that out ;)
2. copy the bash script (zimbra-google-calendar-export.sh) below to your Zimbra server. This will generate the local iCal files.
3. edit the /opt/zimbra/httpd/conf/httpd.conf and add a Options -Indexes entry to the Directory entries below. This will stop Apache from simply listing ALL files in the directory, which is not optimal as it simply exposes all the iCal files to the world, eek!

   <Directory />
       #Options FollowSymLinks
       Options -Indexes
       AllowOverride None
       Order deny,allow
       Deny from all
   </Directory>

   <Directory "/opt/zimbra/httpd-2.2.14/htdocs">
       #
       # Possible values for the Options directive are "None", "All",
       # or any combination of:
       #   Indexes Includes FollowSymLinks SymLinksifOwnerMatch ExecCGI MultiViews
       #
       # Note that "MultiViews" must be named *explicitly* --- "Options All"
       # doesn't give it to you.
       #
       # The Options directive is both complicated and important.  Please see
       # http://httpd.apache.org/docs/2.2/mod/core.html#options
       # for more information.
       #
       # Options Indexes FollowSymLinks
       Options -Indexes
    
       #
       # AllowOverride controls what directives may be placed in .htaccess files.
       # It can be "All", "None", or any combination of the keywords:
       #   Options FileInfo AuthConfig Limit
       #
       AllowOverride None
    
       #
       # Controls who can get stuff from this server.
       #
       Order allow,deny
       Allow from all
    
   </Directory>

4. after making these changes restart the Zimbra server (or just the Zimbra Apache server): zmapachectl stop; zmapachectl start
5. add a crontab entry to regularly export all the Zimbra Calendars, once a day should be suffficient 0 7 * * * root /etc/custom-scripts/zimbra-google-calendar-export.sh > /dev/null 2>&1
6. run the zimbra-google-calendar-export.sh as root and then check /opt/zimbra/httpd/htdocs you should see a listing of *.ics files
7. each ics file uses the Zimbra account ID (which is pretty random) as the file name. Check the /var/log/zimbra-google-calendar-export.log file to work out which Zimbra account corresponds to which .ics file. These file names do not change.
8. the URL for each .ics file will be something like, http://zimbra.xyz.com:123456/1111231a-v476-48g1-e1gc-h1ks921k198s.ics (don't forget your random high port number!)
9. you can now log into Google Calendar:
   1. add a new Calendar
   2. subscribe to a remote URL (again, bummer that Google cannot do URL authentication)
   3. create a new Calendar and supply the magic URL (http://zimbra.xyz.com:123456/1111231a-v476-48g1-e1gc-h1ks921k198s.ics)
   4. Google will now fetch the .ics file and populate your new Calendar
10. check your Android phone's Calendars and you should now see your imported Zimbra calendar. WOO HOO!

Copy the following bash script as zimbra-google-calendar-export.sh and put on your Zimbra server.

Another easy option for individual use or when you don't have root access to the Zimbra server is to:

1. using the Zimbra REST API you can perform a GET request to the Zimbra server and download a copy of calendar just by knowing the required user name and password.
2. so you can do this in a Web Browser: https://somedude:somedudepassword@mail.acme.com/service/home/somedude/calendar?fmt=ics
3. or use wget and a cron job to download the calendar to a remote web server, which Google Calendar can then subscribe too, like: wget --no-check-certificate https://somedude:somedudepassword@mail.acme.com/service/home/somedude/calendar?fmt=ics

This is handy for Geeks who have access to bash and a web server, but not necessarily the Zimbra server.

1. you might have noticed that the above script runs pretty damn slow. This is because with each call to zmmailbox and zmcontrol a new Java VM is instantiated. It would be great if it were possible to instantiate zmmailbox and zmcontrol once and pipe commands to it. This is currently beyond my bash kung fu.
2. it would be better if the calendar was synchronised more regularly with Google. Like hourly instead of once a day. I'm been working on another version which does this using libopensync similar to a method outlined here, http://satyajitranjeev.wordpress.com/2009/08/02/synchronizing-nokia-e-series-with-fedora I have had some success but is still a work in progress.