Jetspeed (as of 1.2b1) implements a subset of RFC 2445 or iCalendar.

From RFC 2445: There is a clear need to provide and deploy interoperable calendaring and scheduling services for the Internet. Current group scheduling and Personal Information Management (PIM) products are being extended for use across the Internet, today, in proprietary ways. iCalendar is an internet RFC that addresses communication between calendaring clients. It defines a set of objects and a method for writing those objects to a character output stream. Any client which understands the iCalendar protocol should be able to share appointment , to-do, and other calendar information with any other calendar client or server.

iCalendar was made a separate module from Jetspeed, since it is a general utility.

Unfortunately, currently it has lost visibility in the jakarta project. If you are interested in contributing to a portable implementation of RFC2445, please contact Jeff Prickett.