You might need to set up a premier support ticket with screen sharing session to have me help with this. You aren't providing enough details for me to help effectively through this forum. I need very detailed descriptions of what you are doing, what you have tried and where you are getting stuck. There are often multiple ways to achieve any given result, so I can't help without specific details.
Your date validation should just be a WHERE clause in your recordset on the add to cart page to only apply the discounts in the current date range.
To get it applied directly to each row, you can just update the TotalPrice calculation in the cart object to subtract the discount column value.