To begin with you will need to have a recordset that select all of the records from this table, you will want to sum the OrderAmount column like this:
SELECT SUM(OrderAmount) AS OrderTotal FROM Orders
I go this from w3schools website, this is their example.
Once you have the recordset setup like this on your results page you will want to make sure that you have your search page setup as well. Next you will apply the DataAssist Search server behavior to the results page. When you are configuring the search criteria you will specify the date column then the date type for the data, for the starting date criteria you would just specify the comparison as greater than, then for the end date you would do the same but choose less than.
Post back if you have any other questions or have any problems with any part of this.