DAViCal

DAViCal is an open source server for storing and sharing calendar data through the CalDAV protocol. It uses PostgreSQL as its data store and makes calendars available for sharing through CalDAV. It is intended for people or organizations that want to run a server for shared calendars rather than rely on a separately managed calendar-sharing service. DAViCal does not replace a specific paid product identified here; it provides the server component for CalDAV-based calendar sharing. One practical limitation is its reliance on PostgreSQL, so it is not database-agnostic and requires a PostgreSQL database as part of the setup. It is also a server rather than a complete calendar application, with its stated role focused on storing and sharing calendar data through CalDAV.