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Depends on how your sheet, table and data are set up.
Can you attaché a workbook with sample data that simulates your application?
I would probably keep a rolling status for each customer that maintains the last purchase date. Then on each current row, if the last date was more than 2 months ago then identify him at that time as new (or whatever you need to do there.)
If you have rows for each customer on each month (and the sample data you gave DOES indicate you do that) then a window_sum(sum([Purchase],-2,0) would indicate that he had a prior purchase in the previous 2 months if the current month is 1 and the window_sum is >1.