Calculating days between clinic visits

Hello,

I have a database with two columns, column A is date of clinic visit, and column B is patient name. I would like to be able to calculate the average days between clinic visits per patient. Im having lots of trouble figuring this out, I would greatly appreciate if anybody could help.

Sample database:

A B
16-Mar-10 Smith, John
10-Oct-12 Smith, John
22-Dec-12 Smith, John
5-Feb-09 Smithers, Jane
24-Nov-12 Smithers, Jane
5-June-11 Springer, Mary
12-May-13 Stein, Jason

It would like to figure out a way for column C to display days between exams for each patient (ie Smith, John would be 939 days between visit 1 and 2, and 73 days between visit 2 and 3).

Thanks to all,