Hi Alberto,
The web is (mostly) a "pull" technology. Meaning the browser asks for a page and that's what it gets. The web (traditionally) had no way to "push" values from the server to the client.
Then along came web sockets (supported in NT9, requires Cryptonite) which allows the browser to "hold" a connection open to the server, and for the server to then send data back down that socket.
So to answer your question simply it's "use web sockets". It's slightly more complicated in terms of using the calendar, and I would likely need to add some code to the calendar to make that a bit easier, but ultimately will be based on web sockets.
cheers
Bruce