Please provide any additional information below.

I will suggest that the GC for v3.x should (or can I say 'must'  ) be in the line of "enhancing" the built-in Calendar and/or "building on-top" of it.

1) That way, those without GC can still see whatever was inputted via GC;
2) GC can use the centralized DB of the built-in calendar;
3) other things as well

For example, GC for v3.x instead of what it is now, adds cross-guild communication. If there is an update, GC writes the update to the built-in Calendar. If someone not using GC updates something via the built-in, someone with GC can push the update to the other guild.

Something like that, and I believe there'll be more useful scenarios in that kind of setup.

Of course, by using the built-in calendar, less coding for you guys

Now on the question of which UI.. I believe GC should have an option of letting the user choose which Calendar UI to use - built-in calendar or GC.

Then those who are going to GC-UI, can also have the features offered by the built-in calendar (like in-game events, centralized DB [mentioned above], etc.)

Just a thought ^_^

I dunno, I think its better that way, win-win for everyone.