Tools question

Hi,

I am a bit lost which calendar tools provide what capabilities
and which ones are currently most actively maintained, etc.
It would be great if someone could point me to the tool(s)
that support the following requirements:

1) I need to maintain a central, shared calendar; it must be queryable
   via HTTP+application/rdf+xml to use the calendar information
   in other application

2) I need to provide an HTML GUI to edit the calendar.


I looked breifly at Sunbird, but I have the impression that it would not
be easy to query its calendars according to the requirement 1), is that
right?

Thanks in advance for any help/clarifications.

Jan