On the piece of paper I was given it had something about it expiring in 24 hours but the actual credit-card-like expiry of MM/YY is next year. I don't know how they can build that expiry day into the credit card-ish information associated with the voucher..
And even if I had managed to input the details into the Starbucks app within 24 hours, how could it then work when I actually used it as payment?

Oh well, lesson learned, I'll try to put it on the app immediately next time. Thanks for responding.