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Proton Calendar vs Fossify Calendar: catalog facts
Proton Calendar	Fossify Calendar
Freemium Free account available, with paid Proton plans and a business version.	Free Free and open source with no ads.
Android, iOS, Web	Android
End-to-end encryption of event details Built-in scheduling for booking meetings	No ads or tracking Syncs with Google, Outlook, Nextcloud and Exchange calendars
Requires a Proton account Works best within the Proton ecosystem	Android only Sync relies on accounts already set up on the device
Downloadable app, Web application, Hosted service	Downloadable app
Closed source	Open source
License not stated	License: GPL-3.0-only
Account needed	No account needed
Not stated if it works offline	Works offline
Official website Full listing and sources Checked September 23, 2026	Official website Full listing and sources Checked October 2, 2026

Catalog facts only. Anything “not stated” is unconfirmed. Check full listings for details.