Protonmail, a Swiss-based, end-to-end encrypted email service, has introduced a calendar in beta:
ProtonCalendar uses end-to-end encryption to keep all your events’ sensitive information private and secure. The event title, description, location, and participants for every event are encrypted on your device before they reach our servers, so that no third party (including ProtonMail) can see the details of your events. Only you will know your plans.
ProtonCalendar is available to all paid ProtonMail accounts using using the ProtonMail Version 4.0 beta.