When the trial court imposes an aggregate sentence comprised of a term previously imposed that is being served and a current term, it must credit defendant with all actual days in custody, whether in jail or prison, up to the sentencing date. Appellant was convicted and sentenced to prison in case “A.” While serving the term on case “A,” he was prosecuted and convicted in case “B,” and the court sentenced him to a single aggregate sentence comprising both cases. When a defendant has served any portion of time on a sentence that is subsequently modified, such time that he has served is to be credited on the subsequent sentence and the court must determine the amount and reflect it on the abstract of judgment. (Pen. Code, sec. 2900.1; People v. Buckhalter (2001) 26 Cal.4th 20.)