ProtonMail introduces ProtonCalendar (beta), a complete analogue of the Google Calendar service with end-to-end encryption.
For now, any paid ProtonMail or ProtonVPN user can try ProtonCalendar (beta), starting from the Basic plan. How to test it: Login to your ProtonMail account (select ProtonMail Version 4.0 beta) and select the calendar in the sidebar.
According to developer Ben Wolford, the release version will be free for everyone.
Since we don't monetize our users, we support our service through subscriptions, and one of the perks of a paid account is early access to new products and features. Once ProtonCalendar is out of beta, it will be available to users with Free plans as well.
Here you can read about the security of the calendar.
The closest free alternative to an encrypted calendar is Nextcloud, which allows you to set up your own cloud service with many useful plugins (including the Nextcloud Groupware calendar). or platform Collaborate Online based on Nextcloud and LibreOffice. But the main problem of such solutions is that the entire headache of setting up, ensuring stable operation, security, updates and backups lies on the user's shoulders. In this regard, ProtonMail offers a turnkey enterprise solution for everyone.
Source: linux.org.ru