Just about. Morgen connects with Google Calendar, Exchange on Premise, Office 365, Outlook, Yahoo!, Apple Calendar, NextCloud, ownCloud, Zoho, Fastmail, and even your own self-hosted CalDAV server.
We don’t currently integrate with Proton Calendar because they don’t have an API available.
If you try to connect a calendar of any kind and it doesn’t work, please
let us know we’ll help.