The release date can change as an inmate accrues new charges. There are no guaranteed release dates when you are the property of the government. There is also a possibility that a different jurisdiction has it's own charge and/or warrant which would extend the stay.
https://www.inmateaid.com/ask-the-inmate/how-are-release-dates-determined-and-can-they-change-from-the-posted-date.amp#answer
Accepted Answer Date Created: February 14,2016