ADD a calculating filed in pivot

[sal21]

About the iamge attached, i need to insert a calculating fild in pivot...
Example:
in E12 the name of filed in this case DIFFERENCE

In E13 diff from C13-D13
In E14 diff from C14-D14
In E15 diff from C15-D15
...

note:
Tks in other case for assistence in my old Query prob but all my query are all without success

[HansV]

Click on AGG or one of the dates of the AGG field.
On the PivotTable toolbar, click PivotTable | Formulas | Calculated Item
Type DIFFERENCE in the Name box.
In the Formula box, enter

='30/12/2010'-'30/01/2011'

Click Add, then OK.

[sal21]

Click on AGG or one of the dates of the AGG field.
On the PivotTable toolbar, click PivotTable | Formulas | Calculated Item
Type DIFFERENCE in the Name box.
In the Formula box, enter

='30/12/2010'-'30/01/2011'

Click Add, then OK.

Error!

[HansV]

I'd have to see the workbook.

[rory]

You are trying to add a calculated field - as Hans said, you need a calculated item.