Hello!
I think this can be done with formulas, but maybe a pivot table or vb would be better.
For the first value in a row, I need to sumif all the values which a) Match the date of the ouput column, and b) have >=3 preceding months.
if you take a look at the attachment:
F3 in green sums colB where colC=F2. It only does so for PersonA and PersonB because E3 is on or after their 3rd month.
G4 applies the same logic, adding PersonC to the mix. (note that PersonA and PersonB are included, since they also meet the definition of 3rd month or later).
and so on
I'm open to suggestions; I can't think of the most efficient way to do this
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