I have a sequence of dates. I want to consider these dates as a group if the difference between them is less than 20 days (The condition).
For example the sequence of below dates can be divided in to two groups :

9/13/2013
9/20/2013
9/27/2013
10/4/2013
11/24/2013
12/1/2013
12/8/2013
12/15/2013
12/22/2013
12/29/2013

1-From 9/13/2013 to 10/4/2013 (The difference between dates is less than 20 days.)

The difference between 10/4/2013 and 11/24/2013 is more than 20 days. As consequence, the second group starts from 11/24/2013

2-From 11/24/2013 to 12/29/2013 (The difference between dates is less than 20 days).

I want to add a column to this table to show these sequence:


9/13/2013 1-Initial
9/20/2013 2
9/27/2013 3
10/4/2013 4-End
11/24/2013 1-Initial
12/1/2013 2
12/8/2013 3
12/15/2013 4
12/22/2013 5
12/29/2013 6-End

How can i do that? Thanks