Cancel Save workflow that checks date field

Hello,

We need a workflow that will cancel a save anytime a sales rep indicates that we have to shop for an item and the customer requested date is less than 14 days. 

 

So far I have written:

 

(PRE('<bShopping_Cust_accepts_min_buy_requirements_ITAG>') = FieldValue('<bShopping_Cust_accepts_min_buy_requirements_ITAG>') OR PRE('<bShopping_Cust_accepts_min_buy_requirements_ITAG>') IS NULL) AND (FieldValue('<bShopping_Cust_accepts_min_buy_requirements_ITAG>')

 

This keeps the record from saving anytime someone checks the box - Shopping.  I need to incorporate the date field [<dDate_Proposal_due_to_customer_ITAG>] and get it to look to see if the date is less than 14 days....if the date is blank or less than 14 days I need the save to be canceled.