Here is an example, it's using the daverage function but the principle is the same for dcount
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If your data is in cells A1:C15
Name Sales Date
Smith 10 1/11/2004
Jones 20 1/12/2004
Miller 5 1/13/2004
Johnson 2 1/14/2004
Smith 5 1/15/2004
Jones 8 1/11/2004
Smith 20 1/12/2004
Johnson 12 1/13/2004
Smith 15 1/14/2004
Smith 6 1/15/2004
Miller 22 1/11/2004
Johnson 11 1/13/2004
Smith 9 1/15/2004
Jones 3 1/17/2004
then in cells(A18:C19)...type this, note: (this criteria definition can be typed any where), I just selected a18:c19
...here is where you can change the search criteria on the fly.
Name Sales Date
Smith >1/13/04
then to average the sales of Smith that occured after 1/13/04, the formula in a cell of your choice
=DAVERAGE(A1:C15,"Sales",A18:C19)