There was a similar question regarding the required Amex spend not that long ago and based on that scenario / answer, I would err on the side of caution and make sure your outbound leg is taken the day before the expiry date. I believe the OP of that question phoned Amex and managed to resolve the issue. However, a 241 voucher is a different ball game.

Welcome to Flyertalk, I'm sure one of the big guns will be along shortly to officially welcome you and provide a more definitive answer.