How to add based on the 4 hour timeperiods

Hello

I am trying to create a solution which compares 4 hour counts of a day with a average of the same 4hour period of time across last 60 days.

_time                   Org   Count

8/19/13 12:00:00 AM      A     5
8/19/13 4:00:00 AM       A     5
8/19/13 8:00:00 AM       A     5
8/19/13 12:00:00 PM      A     5
8/19/13 8:00:00 PM       A     5
8/18/13 12:00:00 AM      A     5
8/18/13 4:00:00 AM       A     5
8/18/13 8:00:00 AM       A     5
8/18/13 4:00:00 PM       A     5
8/18/13 8:00:00 PM       A     5
8/17/13 8:00:00 PM       A     5

As you can see, its not necessary that it will output for all 6 time cycles for a day. Can anyone please help me on creating a logic which can help me in doing mathematical calculations based on the timeperiods. It needs to add 8/19/13 12:00:00 AM with 18 12:00:00 AM, 17th 12:00:00 AM and so on.

Thanks theou