Normally the release date includes the "good time credits", therefore we would not be optimistic that there would be additional reductions unless the inmate becomes a cooperating witness in another case that results in a conviction. At that point the government would likely offer a sentence reduction depending on the value of the information and the level of cooperation.
https://www.inmateaid.com/ask-the-inmate/my-inmates-release-date-is-in-2021-the-set-date-has-been-posted-is-there-any-possibility-due-to-good-behavioretc-that-the-date-can-be-changed#answer
Accepted Answer Date Created: September 30,2015