An online system receives a discount offer from a merchant and displays the offers to users. Some of the discount offers provide for a discount that becomes valid when the discount offer is displayed to the user and remains valid for an offer duration. To implement these discount offers, a timestamp representing the time at which the discount offer is displayed is recorded and encoded. The encoded timestamp is provided to the merchant system when the user makes a purchase. The merchant extracts the recorded timestamp from the encoded timestamp and compares the recorded timestamp to the time of the purchase. If the time elapsed between the timestamp and the purchase is less than the offer duration, then the merchant system applies the discount to the purchase.