Proton Calendar is a privacy-first calendar with end-to-end encryption, ensuring that only you can access your events. Developed by the creators of Proton Mail and based in Switzerland.
Cal.diy is a fully open-source, MIT-licensed scheduling platform forked from Cal.com with all enterprise features removed, self-host your own booking infrastructure with no license key, account, or commercial dependencies.
Tuta Calendar is a quantum-safe, end-to-end encrypted calendar app with a zero-knowledge architecture that hides even your event metadata from the server — free, open source, and available on web, Android, iOS, and desktop.
Mailfence Calendar is part of a secure email and productivity suite based in Belgium. It supports encryption, digital signatures, and privacy-friendly collaboration.
Nextcloud Calendar is a self-hosted calendar solution that integrates with encrypted file storage and collaboration tools. Ideal for users who want full control over their data.
EteSync provides end-to-end encrypted synchronization for calendars, contacts, and tasks. It works with existing calendar apps while keeping all metadata private.
Skiff Calendar was an end-to-end encrypted calendar service focused on privacy and collaboration. The project has been discontinued, but it influenced modern encrypted productivity tools.