Sumproduct and averaging date differential

I am trying to accomplish the following:

A1: Beginning Date.... e.g. 1/1/14
A2: Ending Date..... e.g. 1/31/14

Column A (rows 3:50) - Date (sent out date)
Column C (rows 3:50) - Date (received date)
Column D (rows 3:50) - Date (processed date)

I can get the first part of the sumproduct formula down:

=sumproduct((Sheet1!A3:A50>=A1)*(Sheet1!A3:A50<=A2)*...............rest that I don't know)

Every row will have a date (sent out date) in Column A and Column C and Column D will have data (date) as items are received (Column C) and then moved to processed (Column D). I am trying to find out the average number of days it takes for an item to be sent out (Column A) to processed (Column D) if Column D has a date.

Thank you for any help.

Josh

I have provided a workbook example with 4 rows of data if that will help.

Thank you again,

Josh

An example with array formula.Pl see file.