It kind of seems to me David is looking for a rolling three day average. If today is the third day of the month, say 10/3, then he needs the average for 10/1+10/2+10/3. Tomorrow will be the fourth day of the month so he will need the average for 10/2+10/3+10/4, and so on.
Greg
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The glass is at one half capacity: nothing more, nothing less.