Illustrative example
A buyer discovers three months after purchase that a piece of equipment was misdescribed, but keeps using it while deciding what to do. When they finally ask for their money back the seller says they accepted it long ago.
Section 1691 requires notice of rescission promptly upon discovering the facts, and restoration or an offer to restore. The date that matters is the day the buyer discovered the misdescription, not the day of purchase, and continuing to use the item after that date is what the seller will point to as affirmation.
The parties treat the claim as one for a price reduction rather than rescission: the seller refunds an agreed part of the price and the buyer keeps and continues to use the equipment.