Kronolith is the Horde calendar application. It is currently in the
development stages, and makes heavy use of the Horde framework to
provide integration with other applications.

Right now, Kronolith implements a solid, stand-alone calendar system,
allowing repeating events, all-day events, custom fields, keywords,
and managing multiple users through Horde Authentication. The calendar
API that Kronolith uses is abstracted such that it could work with any
backend, but right now it uses the MCAL calendar library for a backend.

WWW: http://horde.org/kronolith/