Sumx and datesbetween two date columns

Hi AlexanderBP,

Please try this column formula, it results in the following table based on your sample data. 

20DaysPurchases = 
    var custId = [Customer id]
    var firstPurchaseDate  = CALCULATE(MIN([Customer Added Date]), filter(ALL('Table'), 'Table'[Customer id] = custId))+20
return 
    CALCULATE(SUM([Turnover]), FILTER(ALL('Table'), 'Table'[Purchase Date] <= firstPurchaseDate && 'Table'[Customer id] = custId))

Hope this Helps,
Richard
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Hi AlexanderBP ,

The below column works in your scenario, too. The calculation logic is similar to that of richbenmintz.

And if you create it as a measure, it also works.

20DaysPurchases = 
VAR FirstPurchase =
    CALCULATE (
        MIN ( 'Table'[Purchase Date] ),
        ALLEXCEPT ( 'Table', 'Table'[Customer id] )
    )
RETURN
    CALCULATE (
        SUM ( 'Table'[Turnover] ),
        FILTER (
            ALLEXCEPT ( 'Table', 'Table'[Customer id] ),
            'Table'[Purchase Date] <= FirstPurchase + 20
        )
    )

You can check more details from here.

Best Regards,

Icey

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