The expiration date listed on your companion fare discount code indicates the date by which you must redeem your discount code and purchase your fare. Once ticketed, travel can be completed after the expiration date. 12 months from date of issue.
You can view your discount code's expiration date by logging into your My account
profile and viewing the "My discount codes" section. The expiration date cannot be extended, and once your discount code has expired, it cannot be exchanged.