PivotField.BaseField (Excel)

Returns or sets the base field for a custom calculation. This property is valid only for data fields.

This property is not available for OLAP data sources.

With Worksheets("Sheet1").Range("A3").PivotField 
 .Calculation = xlDifferenceFrom 
 .BaseField = "ORDER_DATE" 
 .BaseItem = "5/16/89" 
End With