I decided to add grouping date columns using the format "yyyy-MM" to CC. It will be in the next release (in a week or two). This change obviates the need to create the custom column unless you want a different grouping granularity.
Given that one can now group on dates, I also added the possibility of adding calibre's pubdate and date fields to the list of categories to group upon. The lookup names are "pubdate" and "date".
Apologies for all the posts. I have found that "edits" often aren't noticed.