Which aggregation rule is effective for generating distinct result given complexity of the expressio

Hi!

I have this report that requires computation of Support Request --Days to Close.
However, making it complicated in this computation is the consideration of Task duration (End Time minus Start Time) associated in the SR.

Example:

Support Request#1
SR Open Time : December 1, 2012
SR Close Time: December 20, 2012
DURATION: 19 Days

Within the same SR, there are 2 Tasks:
(1)
Task Start Time: December 5,2012
Task Start Time: December 7,2012
DURATION: 2 Days

(1)
Task Start Time: December 10,2012
Task Start Time: December 11,2012
DURATION: 1 Day

Support Request#2
SR Open Time : January 1, 2012
SR Close Time: January 5, 2012
DURATION: 4 Days


Given the above, the report should generate result as follows:

SR#1 Duration = 19 Days - (2 Days + 1 Day)