We've disallowed Google from searching OnDemand instances by default (AOD-5685), but with a downside: it's now impossible to add Team Calendars calendars to Google Calendars. Google returns an error: "Could not fetch the url because robots.txt prevents us from crawling the url."
This is documented here, the workaround is to add the following line to robots.txt:
We should add this to OnDemand's robots.txt.
Navigate to Confluence administration > Add ons > Team Calendars > Configure > uncheck Disable Private URLs. You will need to copy the URL again as it will change when you make the following change from public to private. Be aware authentication will not be required to access the URL.