I'm attempting to create a report that analyzes multiple fundraising appeals, where each appeal is a group.  Specifically, I would like to count the total number of gifts that have come in after each week of the appeals, using the minimum value of the group.  I hope this sample data illustrates what I am trying to do:

Sample Data
Appeal 1 gifts 4/1/2008, 4/3/2008, 4/4/2008, 4/10/2008, 4/10/2008, 4/15/2008, etc.
Appeal 2 gifts 7/5/2009, 7/6/2009, 7/10/2009, 7/22/2009, etc.

Result
Appeal 1  gifts after 1 week (4/1/2008-4/7/2008)=3, after 2 weeks (4/1/2008-4/14/2008)=5, after 3 weeks (4/1/2008-4/21/2008)=6, etc.
Appeal 2  gifts after 1 week (7/5/2009-7/11/2009)=3, after 2 weeks (7/5/2009-7/18/2009)=3, after 3 weeks (7/5/2009-7/25/2009)=4, etc.

I am having trouble coming up with a way to create these ranges without getting a formula cannot be a range error.  Can anyone help?