Hill v. United States

In 2010, Congress passed a law that reduced the prison sentences for individuals who are convicted of crimes involving “crack” cocaine, which is the most common form of cocaine distributed on the streets. Congress did not specify whether the shorter sentences applied only to individuals who committed crimes involving “crack” after the law went into effect, or whether it also applied to individuals who committed their crimes before the law was passed but were not sentenced until after the law was enacted. By a vote of five to four, the Court held that the 2010 law applies to everyone who was sentenced after the law went into effect, no matter when they had actually committed their crimes.

. - from SCOTUSblog