The Interval() function is just a formatting function, so you don't need it in Sunny's first formula. (Doesn't do anything bad, either...). So if you use
(Max(Order_Date) - Min(Order_Date)) * 24
you will get a number corresponding to the number of hours.
And if you use
Interval(Max(Order_Date) - Min(Order_Date), 'h')
you will get the number of days, but formatted as hours. E.g. if the difference between min and max is exactly 10 days, the underlying value will be 10, but its textual representation will be '240'.