Right-click on the OrderDetails table and select New column.
Enter the following DAX then hit Commit:
Expected Delivery Days = DATEDIFF ( OrderDetails[OrderDate], OrderDetails[ExpectedDeliveryDate], DAY )
A square icon with a sigma symbol should appear next to the name.
This calculated column will give us the number of Expected Delivery Days at the row level. This by itself isn’t a very useful analytical attribute, however, the average Expected Delivery Days would be useful.
Click on the Expected Delivery Days so it is highlighted in a grey shade.
Under Column Tools, Change the Summarization: Sum to Average.