Not sure what Millington is, but a quick good google search produces a land locked naval base??? Go figure...
A lot of the branch specific regulations, in my brief exclusive, tend to overlap. In the Army we use a personnel action form (DA 4187) for a Soldier to request an early separation date the day the are issued their orders which are typically cut 90 days out from the signing of their 199. The signing authority for the DA4187 is the Company Commander (O-3) and can be hand delivered to transitions by the Soldier. Amended orders are cut either same day our by midday the next day. They can be shifted to the left to any arbitrary date, so long as the Soldier can clear post in time. We typically give them 10 business days to clear and despite some 4 day weekends or odd training events, most do not have an issue making these hit times.