Updating Cart Prices when Discount Sales Begin and End

We often run sale promotions, where we discount a group of items at set percentage discount. To do this, we update each item's Base Price field with a "Sale" price. When the promotion is over, we reset the Base Price field to the the normal price.

The issue we have is that any items that were added to the cart during the sale retain the discounted price after the sale ends and prices have been reset. On the flip side, any items that are in a customer's cart when the sale begins keep the normal Base Price, so the customer does not receive the discount.