Notice 2016-66 defines the Notice

Computation Period as the most recent five taxable years of Captive or, if Captive has been in existence for less than five taxable years, the entire period of Captive’s existence. For purposes of the preceding sentence, if Captive has been in existence for less than five taxable years and Captive is a successor to one or more Captives created or availed of in connection with a transaction described in the notice, taxable years of such predecessor entities are treated as taxable years of Captive. A short taxable year is treated as a taxable year.