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vezerid
 
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Default Counting unique records based on date range

aspAddict:

The following formula will work IF your original data in Sheet1 is
sorted by date then cust_id.

In Sheet2!B2 supply the following ARRAY formula:
=SUM((Sheet1!$A$2:$A$3113=A2)*(Sheet1!$B$2:$B$3113 <$B$3:$B$3114))
to be entered with Shift+Ctrl+Enter

HTH
Kostis Vezerides