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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