Calculate Date difference in pivot table

I have a pivot table with irrigation meter readings- (customer number, meter read date, and consumption are the fields). I'm trying to calculate the number of days between meter readings. I put the meter read date in the data area, changed the field settings to calculate the difference from the previous record. I attached a screen shot of the field setting window. I'm not sure I have it right, because the data calculates to "#N/A". Thank you for your time and thought.