If it's telling you a fly by date I'm going to assume we are dealing with an FFC which must be booked and flown before that date and can only be used by the named passenger it was issued to. In contrast, an ETC has an expiration date by when travel must be booked but you can fly after that - you can book anything until the end of the calendar and also it can be used by anyone, not just the person it was issued to.

Canceling within 24 hours never helps you because then it always goes back to the original form of payment so it will go back to your FFC with the original fly by date. If you wait 24 hours to cancel its a YMMV situation - some have reported getting a new FFC with an expiration 1 year out from booking date but others have gotten the same FFC back. In case the latter happens you can try to use the change flight feature and see if you can push the date out past the fly by date and keep doing that until you're ready to book something you're actually going to use.