Thanks for the response! I guess I was referring more to the extensibility of Moodle when it comes to extending or overwriting existing features with more functionality via custom development.
As you mentioned, it's open source, but I would want to avoid modifying the original source since that essentially forks the Moodle source and creates a situation where upgrading Moodle (as new updates come out) is very difficult (if not impossible). For example in Wordpress, you can extend Wordpress to do just about anything you want without having to edit WP source code. I'm hoping there's similar capabilities within Moodle?
Moodle is massively customisable throught its modular design that allows the creation of plugins. You can utterly transform how Moodle looks and behaves without touching the core code. It is what I do for a living for one of Moodle partners, as do many other developers.
Be aware that Moodle is significantly more complex than Wordpress. A fresh install of Wordpress creates around 12 tables (or it did a year or so ago). A fresh install of Moodle installs around 300 tables. The number of tables is not a perfect metric of complexity but it does give you a broad idea.
I am teacher and cannot program anything, even HTML. But I have managed to customize Moodle by hiring a programmer to do what I explain to him/her. I mostly design activity modules, blocks, course formats and question types for quizzes. Those plugins do not touch Moodle core code, but I do have to spend about $500/year to update 5-10 plugins to new versions of Moodle. Things like permissions are very powerful and can be customized within core Moodle interfaces. I don't know about the Calendar block, but as it is a plugin (even though it is a core block), it would be easy to make your custom calendar that would not touch any of the main Moodle core. The only thing I regret is that I don't have much time to document the customizations and therefore share them with the rest of the community, with one exception--the Sharing Cart--which is downloadable from the Moodle plugins database for anyone to use.