Group by on Date

I have a table with Date and unit column. . I want to find sum of unit column
considering 3 days each

User will specify start and enddate

Eg 

Date            Unit
10/1/2011     1
10/2/2011     2
10/3/2011      3
10/4/2011     4
10/5/2011     4
10/6/2011     1
10/7/2011     2 
10/8/2011     3
10/9/2011     1
10/10/2011    1
10/11/2011    1
suppose if user selects date 10/1/2011  to 10/6/2011

then output should be
start date      enddate         unit
10/1/2011      10/3/2011     6
10/4/2011       10/6/2011    9


pls let me know if anybody has any idea.

Thanks in advance 


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