This module is built for one purpose only - to allow yearly date facets to drill-down to month level.
(This functionality was available in Drupal 7 but has not yet been provided in Drupal8.)

The impressive facet_granular_date module does implement this functionality but unfortunately does not work with facet 'pretty paths'.

The idea of this module is to be as simple as possible - there is no need to change the source of your facets, you can just leave the URL processor as 'pretty paths'.