The offence must be judged according to the law in force at the time it was committed.
If, according to the new criminal law in force at the time of the judgement, the act is no longer a criminal offence or is to be judged more leniently, then the new law must be applied; otherwise, the new criminal law has no retroactive effect.
This is one of the fundamental principles of criminal law in a state governed by the rule of law. It can be extremely important, especially in the case of offences whose assessment also depends on the content of other legislation. These are typically economic offences.