The Illinois Department of Corrections calculates prison sentences on a basic formula of divide the total time in half. Limitations to that formula are sentences that require 85% time be served or sentences where the legislature establishes a fixed minimum sentence. An example of a fixed minimum sentence would be the 180 consecutive day requirement for a 4th offender of 625 ILCS 5/6-303 commonly known as driving on a revoked or suspended license.
The Illinois Legislature has proposed the return of additional time credits that are discussed here:
http://www.chicagotribune.com/news/local/sns-ap-il-xgr--earlyrelease,0,3124267.story
Once upon a time the above calculation was slightly different in that the sentence was calculated as divide the total time in half and subtract six months. The new legislation may be a return to that sentencing structure. However, one should note that the IDOC first looks at the statutory minimum before giving any good time credit. Therefore, an individual who receives a one year sentence under the above referenced 180 day minimum 6-303 charge will still have to spend the 180 days in before getting the credit. In other words, 180 days really means 180 days.
People v. CB
CB was involved in a single car accident where his vehicle hit a tree. He was charged with DUI, Driving on a Suspended/Revoked License, and Damage to Public Property. At a bench trial, two Chicago Police Officers testified against CB. Neither officer had any knowledge of how the accident occured, when it occured, or how long after the accident they arrived on the scene. They had a brief conversation with paramedics when they arrived on scene but no information about the accident was discussed (nor would it have been admissable at trial). The officers contradicted themselves as to which officer approached the vehicle first. Beyond that, the officers were impeached about observations that CB could not walk unassisted to their vehicle and that they actually had to lift his legs into their squad car; none of that information was contained in any of their reports. Beyond that, the second officer to testify indicated that she prepared all of the reports related to this arrest but that her partner actually signed them! There was a finding of not guilty on the DUI and the Damage to Public Property, but a finding of guilty on the driving on suspended license (the State refused to dismiss the charges even though CB now has a clear and valid license).