As ROCK doesn’t support installation of 3th party software, it is at this moment not possible to run the Alarm Clock extension on it. Even if there was a way to ‘hack it in’ it still would be gone at the next ROCK update. So the final solution should be that it is an integral part of ROCK, but no clue if and when this will happen.
But as @crieke already said, you can run it on any PC or endpoint in your network that has node.js installed. My current approach for making extensions more accessible for users is to get them pre-installed on servers and endpoints. I’m already in contact with @spockfish to get it included in his RoPieee endpoint image for the Raspberry Pi. The idea is to come up with a system that wraps an extension installer in a Roon extension. This make extensions manageable (install, update, uninstall) from within the Roon interface.