This field in the Purchasing and Receiving window contains the expected shipping date from the vendor. This date prints on the purchase order along with a description of whether the date is a "ship to arrive by this date," or if it is a "ship on this date." Note the following:
There are some rules that are applied when this field is populated that could affect the Due Date.
If you populate the Ship Date with a date that is greater than the Due Date, the Due Date will be changed to match the Ship Date you entered.
This update to the Due Date is applied when you Change or Add the Ship Date..