How can I SUM a numeric field dependent on a date selected?

How can I SUM a numeric field dependent on a date selected?

Hi.  I'm hoping I can get some help here.  I figured out how to aggregate a numeric value and I felt like I was close to my solution, but so far away.  I need to pre-populate a numeric value based on the sum of that numeric value for a specific date selected.

Let me explain:
I'm adding a customer to a delivery log for a specific date.  When adding the customer, I define how many pieces of furniture are being delivered.  Now I want to add a numeric value to the form that gives a sum total of the number of pieces being delivered on that same date so far.  So if a customer was added yesterday for delivery tomorrow and I'm adding a customer for tomorrow, I would automatically see that the customer added yesterday has 5 pieces to be delivered.  And every time I add a customer for delivery on that same date, I would see the total pieces being delivered from all customers on that date.

If I choose to deliver on another date, the same would be true, but only for all records being delivered on that date.

I hope that makes sense and would  be grateful to any suggestions on how to make this work.  :)

Thanks!!!!