While we're on it, can anyone recommend a good open source calendar app with decent web and mobile interfaces? (if there is one) I'm building internal tools fo

While we're on it, can anyone recommend a good open source calendar app with decent web and mobile interfaces? (if there is one) I'm building internal tools for the record label I make music with. We're going to try self-hosting Zulip vs. Mattermark for collaboration, and it would be great to have a calendar that integrates well.