In In re Joyner (1989) 48 Cal.3d 487, which involved criminal proceedings in California and Florida, the Supreme Court established a "strict causation" test for the award of presentence custody credits under section 2900.5.
"A period of time previously credited against a sentence for unrelated offenses cannot be deemed 'attributable to proceedings' resulting in a later-imposed sentence unless it is demonstrated that the claimant would have been at liberty during the period were it not for a restraint relating to the proceedings resulting in the later sentence. In other words, duplicative credits against separately imposed concurrent sentences for unrelated offenses will be granted only on a showing of strict causation." (Id. at p. 489.)
In reaching its holding in Joyner, the Supreme Court examined the purposes for presentence credits: "(1) eliminating the unequal treatment suffered by indigent defendants who, because of their inability to post bail, serve a longer overall confinement for a given offense than their wealthier counterparts and (2) equalizing the actual time served in custody by defendants convicted of the same offense ." (Id. at p. 494.) It noted that both "purposes are concerned with equalizing the treatment of different individuals each convicted in a single proceeding of the same offense or offenses." (Ibid.)
In People v. Bruner (1995) 9 Cal.4th 1178, the Supreme Court applied Joyner's "strict causation" test to a different factual scenario. Defendant Bruner was convicted of a new crime and received a prison sentence "concurrent" to a term he was already serving for violation of his parole in another case. (Id. at p. 1180.) The defendant's "custody as a parole violator was based in part on the same drug incident that led to the later conviction, but also upon additional, unrelated grounds." (Ibid.) The court held: "When presentence custody may be concurrently attributable to two or more unrelated acts, and where the defendant has already received credit for such custody in another proceeding, the strict causation rules of Joyner should apply. Here, defendant received credit for all presentence custody in his parole revocation proceeding, and he has failed to demonstrate that but for the cocaine possession leading to his current sentence, he would have been free, or at least bailable, during that presentence period. Hence, he is not entitled to duplicative credit against the current sentence." (Id. at pp. 1180-1181.)
Bruner concluded that post-Joyner decisions had correctly applied "a general rule," not limited to the dual jurisdiction situation in Joyner, that "a prisoner is not entitled to credit for presentence confinement unless he shows that the conduct which led to his conviction was the sole reason for his loss of liberty during the presentence period." (Id. at p. 1191.) The Supreme Court in Bruner acknowledged the difficulty of satisfying the rule of "strict causation" but explained that "it arises from the limited purposes of the credit statute itself." (Id. at p. 1193.)
In Bruner, the Supreme Court briefly discussed the second sentence of section 2900.5, subdivision (b), which limits credit where a single period of custody is attributable to multiple offenses subjected to consecutive sentences. It noted: "There is no indication the 1978 amendment to section 2900.5, which added the second sentence to subdivision (b), was concerned with concurrent sentences for unrelated conduct imposed in multiple proceedings. By its terms, the amendment does no more than clarify that when consecutive terms are imposed for multiple offenses in a single proceeding, only one of the terms shall receive credit for presentence custody, while leaving undisturbed the accepted principle that when concurrent sentences are imposed at the same time, presentence custody is credited against all." (Bruner, supra, 9 Cal.4th at p. 1192, fn. 9.)