QlikView and Salesforce Question

[#1]

I have the following Load statement:

Load

ClientName

ProductName

PurchaseDate

PurchaseAmount

From ......QVD File.

Here is the Scenario:

     1.  Client A purchases Product 1 on 1/1/2013 and Product 2 on 2/1/2013 and so on. 

What I would like to accomplish is:

     1.  How long did it take Client A to purchase the Product 2nd, 3rd, etc after the first purchase.  This can be in days and everything is based on the first purchase. 

Does this make sense and can it be done easily.

David