In Johnson v. Franzen, 77 Ill. 2d 513, 34 Ill. Dec. 153, 397 N.E.2d 825 (1979), our supreme court reviewed the recent history of sentencing in Illinois and the application of good-time credit to terms of imprisonment. the court stated: "Until February 1, 1978, Illinois had a system of indeterminate sentences in which those committed to the Department of Corrections for commission of a felony were sentenced to minimum and maximum terms of imprisonment. Good-conduct credits were applied to the minimum term to advance the date of parole eligibility and to the maximum to advance the date beyond which a prisoner could not be incarcerated. The Department was required to prescribe, at a rate within its discretion, a schedule of good-conduct credits for good behavior. These were known as 'statutory good time credits.' The Department was also empowered to award good-conduct credits to prisoners who performed work assignments or participated in other Department programs. These credits were known as 'compensatory good time credits.' Effective February 1, 1978, the General Assembly amended the Unified Code of Corrections (Code) and replaced in some instances the indeterminate sentencing system with a fixed or determinate sentencing system. The Code no longer gives the Department authority to award compensatory good-conduct credits [citation] and no longer gives the Department authority to award statutory good-time credit at a discretionary rate. The Code expressly directs the Department to prescribe rules and regulations providing for good-conduct credits on a day-for-day basis." Johnson, 77 Ill. 2d at 516. The Johnson court described two systems for calculating a prisoner's good-time credit. Under the old system (pre-1978), the Department of Corrections (Department) awarded statutory and compensatory good-time credits. Under the new system (since February 1, 1978), the Department awards day-for-day good-time credit. The Court realized that there is a third system now used in some cases based on truth-in-sentencing legislation; however, this system does not affect plaintiff's sentence (see generally People v. Reedy, 186 Ill. 2d 1, 237 Ill. Dec. 74, 708 N.E.2d 1114 (1999)), and therefore we need not discuss it. Johnson addressed the question of which of the two systems it described should be used to calculate good-time credit when a prisoner was sentenced to an indeterminate sentence and served part of his sentence before February 1, 1978, and part of his sentence after that date. Johnson held that the Department should use the old system to calculate such a prisoner's good-time credit for the part of the sentence he served before February 1, 1978, and the new system to calculate such a prisoner's good-time credit for the part of the sentence he served after that date. However, the appellate court's opinion in Williams v. Irving, 98 Ill. App. 3d 323, 53 Ill. Dec. 746, 424 N.E.2d 381 (1981) showed that determining a prisoner's proper good-time credit is not always that straightforward. In Williams, each petitioner was a prisoner who had been sentenced to an indeterminate sentence and had served part of his sentence prior to February 1, 1978, and part of his sentence after that date. Relying on Johnson, the petitioners in Williams claimed that they were entitled to the full amount of the statutory good-time credit that the Department had nominally awarded them when they began to serve their sentences in addition to day-for-day good-time credit for the parts of their sentences they had served after February 1, 1978. The appellate court agreed with the Department's argument that, in some cases, it was appropriate to continue to use the old system for awarding good-time credit to prisoners for both the time they served before February 1, 1978, and the time they served after that date. The appellate court agreed with the Department's argument because it recognized that, in some cases, it was to a prisoner's advantage to do so. It was to a prisoner's advantage because doing so resulted in an earlier expiration of the prisoner's maximum sentence (out date) than if the Department strictly followed Johnson and used both systems. Williams, 98 Ill. App. 3d at 326. The Williams court also decided that when it was to a prisoner's advantage to receive day-for-day good-time for the part of his sentence that he served after February 1, 1978, i.e., to use both systems, such a prisoner was not entitled to the full amount of the statutory good-time credit that the Department nominally awarded to him when he began serving his sentence. Rather, such a prisoner was eligible only for a pro rata share of the statutory good-time credit that was calculated under the old system. This pro rata share was to be based on the actual time the prisoner served before February 1, 1978. Williams, 98 Ill. App. 3d at 328.