Dave,
Here is what I had to do. It comes with no warranty.
1. Restore that user's mailbox to a new user account. In this way user's existing mailbox will not altered.
$ zmrestore -a accountname -ca -pre newaccountname password -rf
2. Check if you can see appointments in the calender of the new account.
3. If you found all calender data, save it in a ".ics" file by entering following url in browser.
http://server/zimbra/home/username/calendar.ics - replace server with your servername.
4. Place the .ics file on the server i.e. /tmp/Calendar.ics
5. Run: curl -u username

assword --data-binary @/tmp/new.ics
http://server/service/home/username/calendar?fmt=ics -replace username and servername with your values.
Check following links for more information about importing/exporting ics files. Please let us know for further clarification.
User Migration - Zimbra :: Wiki User Migration - Zimbra :: Wiki
Good Luck!