Oracle 18c

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create table ordersales(shippingdate date, packingdate date, orderdate date);

insert into ordersales values(
to_date('03/04/2005','MM/DD/YYYY'),
to_date('02/03/2004','MM/DD/YYYY'),
to_date('01/02/2003','MM/DD/YYYY')
);
1 rows affected

SELECT NVL(TO_DATE(ShippingDate, 'yyyy-MM-dd'),TO_DATE(PackingDate, 'yyyy-MM-dd')) - TO_DATE (OrderDate, 'yyyy-MM-dd') FROM OrderSales;

NVL(TO_DATE(SHIPPINGDATE,'YYYY-MM-DD'),TO_DATE(PACKINGDATE,'YYYY-MM-DD'))-TO_DATE(ORDERDATE,'YYYY-MM-DD')

792

select coalesce( ShippingDate, PackingDate) - OrderDate from ordersales

COALESCE(SHIPPINGDATE,PACKINGDATE)-ORDERDATE

792