create a view of events grouped by month in Drupal 8

Here is my page:
http://dev-swedishmuseum.pantheonsite.io/events

I have a grouping field that pulls in the date and is formatted to just display the month. BUT, of course, the data for the full date is still there, so it groups by specific date, not month. Is there some way to rewrite the output to change the granularity?

I would like to avoid twig template work, but if that’s the answer any tips are welcome !

[ Oddly I found this documentation post which basically described what I am trying to do but in Drupal 7. (I did the Custom Date Type approach). I’m kind of surprised that would work in Drupal 7, I would have thought the full date data was still in the field no matter what the display.
https://www.drupal.org/docs/7/theming/howto/create-a-views-event-list-gr… ]

Drupal version: 


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