In In re Estrada (1965) 63 Cal.2d 740, the court considered whether a statute mitigating punishment for escape should be applied retroactively to a defendant who escaped before the effective date of the mitigating statute. The statute was silent as to retroactive application. (Estrada at p. 744.) The defendant was committed to a rehabilitation center after a narcotics violation, and he later escaped. He was convicted of escape without force or violence in violation of Penal Code section 4530. (Estrada, supra, 63 Cal.2d at pp. 742-743.) At the time Estrada committed the offense, Penal Code section 3044 provided that a person who was convicted of violating Penal Code section 4530 could not be granted parole until such time as he had served at least two calendar years from and after the date of his return to prison following the conviction. (Estrada, at p. 743.) After Estrada committed the offense, but before he was convicted and sentenced, Penal Code sections 3044 and 4530 were amended, and it was conceded that a defendant convicted of escape without force or violence could be eligible for parole in less than two years. (Estrada, at pp. 743-744.) The court held that the amended versions of Penal Code sections 3044 and 4530 applied to Estrada. (Estrada, at pp. 744-745, 748, 751.) The court reasoned that "'a legislative mitigation of the penalty for a particular crime represents a legislative judgment that the lesser penalty or the different treatment is sufficient to meet the legitimate ends of the criminal law.'" (Id. at p. 745.) According to Estrada, a statutory amendment reducing punishment for a crime or changing procedure in favor of a defendant should be given retroactive effect as to cases that have not reached final judgment. (Id. at pp. 744-745.) In reaching its holding, the Estrada court explained: "When the Legislature amends a statute so as to lessen the punishment it has obviously expressly determined that its former penalty was too severe and that a lighter punishment is proper as punishment for the commission of the prohibited act. It is an inevitable inference that the Legislature must have intended that the new statute imposing the new lighter penalty now deemed to be sufficient should apply to every case to which it constitutionally could apply. The amendatory act imposing the lighter punishment can be applied constitutionally to acts committed before its passage provided the judgment convicting the defendant of the act is not final. This intent seems obvious, because to hold otherwise would be to conclude that the Legislature was motivated by a desire for vengeance, a conclusion not permitted in view of modern theories of penology." (Estrada, supra, 63 Cal.2d at p. 745.) Thus, "where the amendatory statute mitigates punishment and there is no saving clause, the rule is that the amendment will operate retroactively so that the lighter punishment is imposed." (Id. at p. 748.) Relying on Estrada in People v. Doganiere (1978) 86 Cal.App.3d 237, 240 Fourth Dist., Div. Two, the Court held that amendments to section 2900.5, providing credit for section 4019 conduct credits, were retroactive. This holding was based on the premise that there is no legal distinction between decreasing the maximum sentence for a crime and increasing presentence credits because both mitigate punishment. Despite numerous cases applying Estrada, the California Supreme Court has not held that increases to the custody credit scheme constitute mitigation of punishment. Rather, our Supreme Court has consistently characterized the custody credit scheme as a means of encouraging and rewarding behavior. (See People v. Brown (2004) 33 Cal.4th 382, 405 (Brown).) We thus conclude the Estrada exception to prospective application of a new or amended statute does not apply, and there is no presumptive retroactivity. (See In re Kapperman (1974) 11 Cal.3d 542, 546.) In re Estrada explains that, while it is presumed under common law that the Legislature intends a statute to operate prospectively, this rule of construction should "be applied only after, considering all pertinent factors, it is determined that it is impossible to ascertain another legislative intent." (Estrada, supra, 63 Cal.2d at p. 746.) Accordingly, where, as here, a court is called upon to determine whether an amended statute operates prospectively or retroactively, it must attempt to ascertain the legislative intent behind the amended statute based on "all pertinent factors." (Ibid.) In Estrada, the defendant, an inmate, was convicted of escape without force or violence in violation of section 4530. (Estrada, supra, 63 Cal.2d at p. 743.) When the defendant committed this crime, section 3044 provided that a person convicted of escape under section 4530 could not be paroled until he or she had served at least two calendar years, calculated from the date of his or her return from prison after an escape conviction. After the defendant committed the crime but before he was sentenced and convicted, sections 3044 and 4530 were amended, such that a person convicted of escape without force or violence could be eligible for parole in less than two years. (Id. at pp. 743-744.) The California Supreme Court held that the amended versions of sections 3044 and 4530 should apply to the defendant because "a legislative mitigation of the penalty for a particular crime represents a legislative judgment that the lesser penalty or the different treatment is sufficient to meet the legitimate ends of the criminal law." (Id. at p. 745.) In sum, a prisoner escaped from the California Rehabilitation Center. After his capture, but before he was convicted and sentenced for his new crime, sections 4530 and 3044 were amended to reduce the penalty for nonviolent escape from two years to one year and to shorten the period of time before an escapee could be eligible for parole. (Estrada, supra, 63 Cal.2d at pp. 742-743.) Reasoning that "when the Legislature amends a statute so as to lessen the punishment it has obviously expressly determined that its former penalty was too severe and that a lighter punishment is proper as punishment for the commission of the prohibited act," the court ruled that the ameliorative statutes are retroactive. (Id. at p. 745.) The court thus created an exception to the common law rule of statutory construction (enacted in 1872, codified in 3), that no part of the Penal Code is retroactive "unless expressly so declared."