There is a general presumption that legislation is intended to operate prospectively, that is, from the time of its commencement, rather than retrospectively to things that happened in the past.
Great uncertainty and possible injustice would result if it were too easy to interfere with past legal rights and obligations.
The presumption does not apply to purely procedural rules, that is, rules which determine the processes by which rights are enforced. But it does apply wherever legislation affects legal rights that are enforceable by bringing an action. In these circumstances, if a legislature wishes to pass retrospective legislation, it must do so expressly, or by necessary implication.