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

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