I have the source for the Enterprise Events module.  It's pretty stable and supports most common calendar functions including reoccuring events.   It's a DNN 3.x module, but it's very stable. This could be a good place to start.

I'm not sure of the status of the code as the original developer, Wayne Wallace(?) at DNNWorx.com, is now longer on line.   I'll take a look at the code and see if there are any copyright notices.

GregC

The Enterprise Events module is written in C#, is a direct port of an open source DNN2 module (is that old enough to call it venerable? ;-).  There is no copy right notice in the code so I'm not sure if that makes it open or closed, however, the last developer did supply the code to the community to further develop so I believe that he intended it to be open source.

I will make the code available on my site and will post the link later (as soon as I can get it uploaded and set up).  I will not support it, just make it available for the community.

As a great credit to the DNN development team and to the all the developers of the EnterpriseEvents module, it still works in DNN 4.8.1. 

Greg