Originally Posted by
findark
December 31, 2022
(Yes, seriously - you do not need to do it before departure, although it would probably be a good idea.)
IIRC the deadline is scheduled departure time for award tickets. I'm not sure whether this applies to nonrefundable tickets too; early in the COVID-19 pandemic, DL was encouraging people just to no show rather than contacting customer service and DL was automatically turning those tickets into eCredits.