I'm not familiar with php but here is an alternative way to solve the issue. In case someone like us still using 1.x Xenforo
1. Open library/Waindigo/CustomDate/CustomField.php
2. Delete the code below
I wonder this custom field is REQUIRED to pick up a date (not type in) so there is no chance to make it "wrong", so no need for "verifyDate". In case if you delete the correct date format, this field will not show up after save the post. No DATE data will be write to the database, sure there is no error log.