One level of the date tree Items.dates: the nodes, each the run of items it spans, in the order the items are listed.
The part this level groups by.
The nodes. A run's value is the group item selecting exactly its items: the parts down to this level ({ group: 'month', year: 1995, month: 10 }).
value
{ group: 'month', year: 1995, month: 10 }
One level of the date tree Items.dates: the nodes, each the run of items it spans, in the order the items are listed.