What is the test for prospective only application of the Prospective Only Application of the Criminal Code Amendment?

We conclude further that prospective-only application of the amendment does not violate appellant's equal protection rights. Because (1) the amendment evinces a legislative intent to increase the incentive for good conduct during presentence confinement, and (2) it is impossible for such an incentive to affect behavior that has already occurred, prospective-only application is reasonably related to a legitimate public purpose. (People v. Hofheiser (2006) 37 Cal.4th 1185, 1200 [legislative classification not touching on suspect class or fundamental right does not violate equal protection guarantee if bears a rational relationship to a legitimate public purpose].)