The current calendar plugin has no connection to calendars which are managed outside of Discourse. In our case we use a radicale server ( https://radicale.org/v3.html ) to manage users calendars which can then be shared on various devices and applications. It would be a huge increase of functionalitiy if the Discourse calendar plugin is able to read data from a caldav ressource and display its content dynamically inside of a topic.
I am not a ruby programmer, but this package may be useful for that (they also use Radicale as a test environment):
https://github.com/pat/calendav
Discuss this on our forum.