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 is going to be used in Jetspeed as a basis for a web based calendar. A decision was made by the author to go with the iCalendar protocol rather than hack together a calendar we might outgrow. At first there will be no support for writing a valid iCalendar stream. The iCalendar objects will simply provide support for web-based calendaring, but as time moves on we will add support for writing the iCalendar protocol.