I have been tasked to put a buffer time to allow modifying this deadline field.
E.g
After confirming the SO. Sales team could adjust and adjust the deadline date, as long as they are allowed to. This needs to put a buffer time so that they couldn't just it if they can't commit with the delivery date. So, after they confirm the Qoutation, the delivery date will count 7 days they could edit it, and after 7 days, they are not allowed to change it anymore.