I have Google Alerts set up to email me any new content for "ExtJS" every week. I think that's how I came across Extensible - a professional 3rd party ExtJS component developer. Just take a look at their Calendar component demos:
- Native ExtJS components
- Professional 3rd party components (Extensible is first one I've seen)
- User extensions
- Your own code
The further away you are from #1, the more likely that code is to have errors, whenever new ExtJS versions ship. I even ran into it with native ExtJS date inputs when they went from 4.0.2 to 4.0.7 - getValue() format became different and I had to refactor some code. That is why I welcome professional 3rd party extensions and would be glad to pay money for them.
Currently the Calendar component is the only offering in the Extensible library, but I hope to see that change soon. I envision Extensible becoming something similar to what DevExpress became to C#. Give 'em a try!