In france, per article 112-1 du code pénal:
Sont seuls punissables les faits constitutifs d'une infraction à la date à laquelle ils ont été commis.
Peuvent seules être prononcées les peines légalement applicables à la même date.
Toutefois, les dispositions nouvelles s'appliquent aux infractions commises avant leur entrée en vigueur et n'ayant pas donné lieu à une condamnation passée en force de chose jugée lorsqu'elles sont moins sévères que les dispositions anciennes.
Which DeepL translates into:
Only acts constituting an offence on the date they were committed are punishable.
Only the penalties legally applicable on the same date may be imposed.
However, the new provisions apply to offences committed before their entry into force and which have not given rise to a conviction that has become res judicata when they are less severe than the old provisions.
This can be summarised as "non-rétroactivité de la loi pénale", which means you cannot be charged for something you committed before the law came to exist.
As far as I know, there are only two exceptions to that:
- If you were in the process of being prosecuted for something which was already forbidden and a new law decreed a reduced sentence, you shall benefit from it. Example: if you were to be condemned to death sentence, and a new law abolished it, then you will no longer be sentenced to death.
- Crimes against humanity is a notion that came to existence only after the events of World War II. Even so, you can be prosecuted for crimes you committed even before the laws existed., per article 213-5 du code pénal:
L'action publique relative aux crimes prévus par le présent titre, ainsi que les peines prononcées, sont imprescriptibles.
Translated by DeepL into:
There is no statute of limitations on the prosecution of the crimes provided for in this Title, nor on the penalties imposed.