The number a judge says in court is rarely the number of days a person serves, and the difference is governed by a set of rules almost nobody explains at sentencing. This article walks through what actually determines time served: how a sentence is computed from the date it begins, what credit is given for time already spent in custody, how good time and earned time credits accumulate and how they are lost, and how release to a halfway house or home confinement shortens the time actually spent inside. It explains the difference between parole systems and determinate systems, why the federal system abolished parole and what replaced it, and how supervised release and probation revocations can add time after release. Separate parts cover the presentence report, which is the most consequential document in the case; concurrent and consecutive sentences; mandatory minimums and the narrow paths around them; and the compassionate release and sentence reduction mechanisms that exist. A final part covers what families can do while someone is inside. Examples run throughout.