I have this table with (cumulative) sales by day:

01/18/2013            11,750

01/14/2013            12,000

01/13/2013            11,500

I need to calculate automatically the difference by day so that the table would look like:

PLS NOTE

11,750-12,000 = -250

12,000-11,500 = +500

I do not have a record every day. I need to take the latest record (by date) and subtract from it the second last record (by date).