compare values in current and previous row

Hi


I want to know intervals of purchases. For example, I have records like

- user01,2012/01/01
- user02,2012/01/02
- user01,2012/01/04
- user02,2012/01/06

How I can add "days since the last purchase"?  like

- user01,2012/01/01,X
- user02,2012/01/02,X
- user01,2012/01/04,3days
- user02,2012/01/06,4days