We can get the solution for your situation by using the Windows Calculations.
1) Drag the the Date Field, then show the data for Year, Month combination
EX: Year, Month
2) create a calculated field,
3) Use the above calculated field on the view.
WINDOW_AVG(expression, [start, end])
Returns the average of the expression within the window. The window is defined by means of offsets from the current row. Use FIRST()+n and LAST()-n for offsets from the first or last row in the partition. If the start and end are omitted, the entire partition is used.
For example, the view below shows quarterly sales. A window average within the Date partition returns the average sales across all dates.
I am sharing my thoughts with out checking since i am answering from my office & don't have Tableau Environment here.
Thanks for the reply , late response from my side due to i'm on vacation by that time.