retrospective
adjective
LAW uk/ˌretrəʊˈspektɪv/ us/ˌretrəˈspektɪv/if a law, decision, etc. is retrospective, it has effect from a date in the past before it was approved:
The new law will not be retrospective.
He described stamp duty on property transactions as 'an unfair and retrospective tax' on long-term savers.
See also
retroactive