Presentence credits are governed byPenal Code sections 2900.5 and 4019. Section 4019 provides that a defendant "confined in a county jail . . . following arrest and prior to the imposition of sentence for a felony conviction" is entitled to receive presentence credits calculated pursuant to a formula set forth in the statute. Those credits "shall be deducted from his or her period of confinement . . . ." Section 2900.5, subdivision (a) provides: "In all felony . . . convictions, . . . all days of custody of the defendant, . . . including days credited to the period of confinement pursuant to section 4019, shall be credited upon his or her term of imprisonment . . . ." for purposes of section 2900.5, "'term of imprisonment' includes . . . any period of imprisonment and parole, prior to discharge, whether established or fixed by statute, by any court, or by any duly authorized administrative agency." ( 2900.5, subd. (c).) Under section 2900.1, "where a defendant has served any portion of his sentence under a commitment based upon a judgment which judgment is subsequently declared invalid or which is modified during the term of imprisonment, such time shall be credited upon any subsequent sentence he may receive upon a new commitment for the same criminal act or acts." In People v. Buckhalter (2001) 26 Cal.4th 20, at pages 32-33 (Buckhalter), the court explained how section 2900.1 differs from the statutes governing presentence detention: "The sentence-credit statutes make only one express reference to a sentence modified while in progress. Section 2900.1 provides simply that 'where a defendant has served any portion of his sentence under a commitment based upon a judgment which judgment is subsequently declared invalid or which is modified during the term of imprisonment, such time shall be credited upon any subsequent sentence he may receive upon a new commitment for the same criminal act or acts.' (Italics added.) "The sentence-credit statutes do not define the phrases 'prior to sentencing' ( 2900.5, subd. (d)) and 'prior to the imposition of sentence' ( 4019, subd. (a)(4)), nor do they expressly exclude the possibility that time in custody pending a sentence remand might be presentence time. Nonetheless, it appears clear in context that the quoted language refers to confinement prior to the time the defendant was originally sentenced and placed in the custody of the Director to commence service of his term. the statutory scheme, read as a whole, plainly contemplates that once sentenced, committed to prison, and delivered to the Director's custody, a felon remains in that status, serving a term of imprisonment, until lawfully released, and earns credits against the sentence, if at all, only pursuant to the laws specifically applicable to persons serving terms in prison. (But see fn. 10, post.) "In particular, we note the manner in which the credit statutes treat modification of a sentence already commenced, as distinct from presentence detention. Emphasis added. on the one hand, section 2900.5 provides that when the defendant 'has been in custody' (id., subd. (a)) 'prior to sentencing' (id., subd. (d)), the trial court must calculate and award, in the abstract of judgment (ibid.), 'all such days of custody, . . . including days credited to the period of confinement pursuant to section 4019' (id., subd. (a), italics added). On the other hand, section 2900.1 provides that when, after the defendant has already been committed and begun to serve his sentence, the judgment is modified 'during the term of imprisonment,' 'such time' already served must be credited against the subsequent sentence. "Section 2900.1 thus speaks in terms of a prison sentence already in progress, and, in contrast with section 2900.5, it omits reference to presentence good behavior credits under section 4019. The implication is that once the defendant is committed to prison, his custody is thereafter considered service of his sentence, and a remand with respect to a sentence the defendant is already serving does not render him eligible for credits of the presentence kind." (Buckhalter, supra, 26 Cal.4th at pp. 32-33.)