To add subtotals to a date field in a List:
1. Copy the Date column and insert it twice.
2. Change the titles for the two new columns to Month and Year.
3. Select the Month column, and press Ctrl+1.
4. Select the Number tab, select Custom, and in the Type box, type MMMM.
5. Repeat steps 3 and 4 for the Year column, and in the Type box, type YYYY.
6. Sort the List by the Year column.
7. Add Subtotals to the List.
Screenshot // Adding Subtotals to a Date Field