The Date Required is set at sales order level in the header and the Date Expected can be changed when you hit purchase on the PO, you will see a pop up where you can change the date expected.
If it was an EDI order the date set will be ignored as the vendor will feedback a date expected.
You can take it further with the date expected and have it set at item level on the PO (so every item could have a different date). On the PO double click the item and in the extensions tab change the date.