How to calculate average days difference

// Obiviously, the field Paid Document No in
// Purchase Payments must be hidden. Slicing
// should always be done via dimensions, never
// fact tables with the sole exception of
// degenerate dimensions. If you stick to this,
// the measure should always work correctly.

Payment days =
AVERAGEX(
    DISTINCT( 'Purchase Invoices'[Document No] ),
    CALCULATE(
        var MaxPaymentDate = 
            MAX( 'Purchase Payments'[Payment Date] )
        var DocumentDate = 
            selectedvalue( 'Purchase Invoices'[Document Date] )
        var DateDiff_ =
            datediff(
                DocumentDate,
                coalesce( MaxPaymentDate, UTCTODAY() ),
                DAY
            )
        return
            DateDiff_
    )
)