The Payment date (due date) is automatically calculated by adding the number of days (entered in the "Due date" field on the Accounting information tab of Debtor accounts / Accounting information tab of Creditor accounts) to the date of the document (i.e. invoices, credit notes, purchase and supplier return documents) and the transaction dates entered in batches.  

To adjust payment date:

1.On the Action ribbon, select Open item link. The Totals tab of the "Open item link" screen will be displayed. 

2.Click on the Transactions tab. 

3.Select the filter options and click on the Filter button to populate the transactions according to your criteria.

4.Select the account and right-click to select the "Adjust payment date" option from the context menu. The "Adjust payment date" will display the current payment dates in the "From" (disabled) and "To" fields. Select or enter the new payment date in the "To" field. open-item-adjust-payment-date

5.Click on the OK button. The new payment date will be displayed in the "Due date" column.