I copied and pasted Pats syntax and I still got a argument error
DaysOccupied: Iif(IsNull([DateOut), DateDiff("d", [DateIn], Date()), DateDiff("d",[DateIn],[DateOut]))
This was my original sql
SELECT tblTenant_Property_Bookings.BookingID, tblTenant_Property_Bookings.PropertyID, tblTenant_Property_Bookings.TenantID, tblTenant_Property_Bookings.DateIn, tblTenant_Property_Bookings.DateOut, DateDiff("d",[DateIn],[DateOut]) AS DaysOccupied, tblTenant_Property_Bookings.ExpectedExitDate, tblTenant_Property_Bookings.[4WeekLetter] FROM tblTenant_Property_Bookings ORDER BY tblTenant_Property_Bookings.DateOut;