The expiration date should be the last available redemption date; that is, the last day you can use it to pay for travel (on any available future date). Be sure to read any applicable terms which may have been included with the voucher, as there have been modifications to the change/cancellation policy in the past year.
In any case, don't wait until the last day to redeem the voucher in the event there are unexpected events or possible glitches in the system which make redemption problematic. Unless otherwise stated, you can always book a reservation using the voucher and change it later if necessary.