This was answered by Eric Snyder:
Currently, the source code for Team Calendars in not publicly available. We are tracking requests for the the source distribution here:
Please, feel free to vote and/or comment on this issue.
The source code is now available for Confluence Team Calendars starting with version 5.1.7 and above.
Commercial, Academic and Starter license holders (named technical or billing contacts) can access the product source code through their My Atlassian account.