add calculations to a pivot table

I have a column that shows times of transactions. I would like to add a field that calculates the gap between those two times. It would be like c6-c5= whatever and on down the data set. How can I do this?

Delete Calculated Field "Gap"
Instead create a new field "Gap" in your data with the formula "=VALUE(B2)", as the "Conf.t." field are not numbers
Pull this "Gap" field to Values section of the PivotTable
=> Summarize Values By "Sum"
=> Show Values As "Difference From"
=> Base Field: "Conf.t.", Base Item: "previous"
=> OK