Federal and state constitutional prohibitions against ex post facto laws are not violated by the application of Proposition 21 (Welf. & Inst. Code, sec. 777) to probation violations occurring after its effective date, even though the crime for which probation was granted predated March 8, 2000. The juvenile court did not err in admitting reliable hearsay consisting of a probation officer’s report recounting statements by the victim Ivan that he had been harassed and threatened at school by the minor. There was good cause for Ivan’s unavailability on the ground that his presence would pose a risk of harm to him due to appellant’s threats.