Picking up on your reply to my post https://www.odoo.com/es_ES/forum/ayuda-1/question/odoo-9-how-set-default-order-date-value-98420 we both have the same problem in that somehow we have set the date in question as a user defined default (normally only possible while in developer mode?)
I have a partial resolution go to Settings > Technical > Actions > User Defined Defaults there is a date_order field if you edit this and clear default value when follow sequence Purchase > Purchase Orders > Create the date will now come up blank.
I do not know the setting to default to current date and if you delete order_date then the system crashes on Purchase > Purchase Orders > Create
If you find out anymore let me know on my question.