They could also go by when the member id was made so only people who made it before the 16th can register. They had to open member IDs because of Wondercon since its necessary to have one to buy a badge online.
This is true, and last year since Member ID was newish for Wonder-Con, registering with a Member ID was OPTIONAL: meaning Member ID sign-ups could be closed and you could still buy a Wonder-Con pass (if I remember correctly, if you didn't buy a Wonder-Con badge w/a Member ID a prompt told you that it was required for all future events and you clicked, "I'll sign up at a later date" or something at the end).