Hierarchical date granularity facet

Enable the Date item processor, select the granularity and enable Hierarchy:

Enable Use hierarchy and select Date item hierarchy as hierarchy type:

Under Advanced settings ensure that Build hierarchy tree is executed before the Date item processor:

Now the result look like this:

The facet's Operator should be set to AND if you want selection of child facets to narrow the list of results. The OR Operator will cause the selection of child facets to include all parents' results.