18.6 Retrospective Legislation
From "The Rule Of Law" by LJM Cooray

Laws should apply prospectively and not retrospectively. A person should never be made to suffer in law (criminal or civil) for an act which was not unlawful when he committed it. Retrospective legislation destroys the certainty of law, is arbitrary and is vindictive, (being invariably directed against identifiable persons or groups). Such laws undermine many characteristics of the rule of law.