It's not missing from all parts of the OS. For example, you can still set calendar appointments that are in December. One example of something you can't do is set anniversaries or birthdates for contacts that are in the month of December.
Google is aware of the issue, and a fix is apparently already available and reviewed. When Android 4.2.1 will roll out is still unknown, though.
Don't worry, Android fanboys. Apple apparently did something similar, so we can all laugh about our respective platforms letting something out into the wild that QA should definitely have found.
As to whether or not the bug was simply let out because someone was watching too much of "2012," Google has not responded to our inquiries, as of yet.