Per the strict terms of the certs, you must fly by the expiration date.
As a practical matter, has anyone booked a flight, applied an RUC/GUC that cleared instantly, gotten the ticket reissued (a critical piece here), and flown up front after the cert expiration? That's the way the old SWUs used to work. And I know just because the T&C say something doesn't mean DL's best-in-class IT enforces it.