Refer to the Order > On Order tab. This date is not editable because it is a display of the Expected Receival Date on the PO. Therefore any changes to the date on the PO will be reflected here.
What if your stock is arriving by shipment?
You can also see this on the Order > On Order tab. Just expose the ShipmentETA column on that tab.
This date is coming from the Receipt Date on the linked shipment.