I think the example data provided is weak and doesn't cover realistic possible examples, and so I ask for clarification from @Julia5, what do you want to happen if these are the dates for one customer?
01JAN18
09JAN18
14JAN18
01MAR18
05MAR18
Should the output include 01MAR18 and 05MAR18, because they are more than 30 days from the start? Or should the output include 01MAR18 but not 05MAR18 because 05MAR18 is less than 30 days from the previous purchase while 01MAR18 is more than 30 days from any previous purchase?
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