Problem with DATEDIFF and computed columns

Hi

I have the following code on MS SQL Server 2012:

ALTER VIEW [dbo].[ExtendedOrderHeaderView]
AS
SELECT        H.*
	,dbo.PreviousOrderDate(H.custID) as [PreviousOrderDate] 
	,RealOrderCount = (SELECT count(o.orderno) FROM OrderHeaders O WHERE (O.custid=H.CustID) and (OrderType=0))
	,RealLastOrderDate = (SELECT max(o.createdat) from OrderHeaders O WHERE (o.custid=H.custid) and (OrderType=0))
	,DaysSincePreviousOrder = DATEDIFF(DAY,h.[CreatedAt],[PreviousOrderDate])

FROM            dbo.OrderHeaders as H
GO

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It gives an Invalid column name 'PreviousOrderDate' on the DATEDIFF when trying to compile. It accepts the table field, but not the computed field name