Next to Last Date

I have a table that tracks each customers visit to my store. I hold
promotions on occaision that bring customers that haven't visited my store in
awhile. When I hold a promotion, I'm trying to determine the number of days
since the last visit for each customer. For example, here is a sample
customer:

Account_No Visit_Date
1234 4/19/10
1234 4/10/10
1234 3/10/10
1234 2/13/10

This customer generally visits once a month. Assuming that I held a
promotion on 4/19/10, I want to calculate the number of days since the most
recent visit, in this case being 4/10/10 (9 days).

The goal is to see if I have affected their trip pattern by holding the
promotion and if I was able to generate an incremental trip by holding the
promotion.