This example sets the data field in the PivotTable report on Sheet1 to calculate the difference from the base field, sets the base field to the field named "ORDER_DATE," and then sets the base item to the item named "5/16/89."
With Worksheets("Sheet1").Range("A3").PivotField
.Calculation = xlDifferenceFrom
.BaseField = "ORDER_DATE"
.BaseItem = "5/16/89"
End With