If the inmate release date is 9/15/24 when would they get out in reality? With all the federal credits?

If the inmate's posted release date is 9/15/24, it has likely already been considered including the good-time credit given at the time of incarceration by the FBOP (automatic 15%). An inmate can only lose part or all of that good time, not gain more ...unless they decide to cooperate and give someone up to get some time shaved.