In any action pursuant to the revised Kansas juvenile justice code in which the juvenile is adjudicated upon a plea of guilty or trial by court or jury or upon completion of an appeal, the judge, shall direct that, for the purpose of computing the juvenile’s overall case length limit and, if incarcerated, sentence and release, eligibility and conditional release dates thereunder, that such sentence is to be computed from a date, to be specifically designated by the court in the sentencing order. If incarcerated, such date shall be established to reflect and shall be computed as an allowance for the time which the juvenile has spent incarcerated pending the disposition of the juvenile’s case. In recording the date of commencement of such sentence, the date as specifically set forth by the court shall be used as the date of sentence and all good time calculations authorized by law and all earned time calculations authorized by law are to be allowed on such sentence from such date as though the juvenile were actually incarcerated in a juvenile correctional facility.