PART II
INTERNATIONAL CRIMES AND OFFENCES AGAINST THE ADMINISTRATION OF JUSTICE
7. War crimes -
(2) For the purposes of this section, a “war crime” means an act specified in article 8(2) of the Statute and any other act committed during an armed conflict which, at the time and in the place of its commission, constitutes a war crime according to customary international law or conventional international law applicable to armed conflicts, whether or not it constitutes a contravention of the law in force at the time and in the place of its commission.
Non-retroactivity
Nullum crimen sine lege
EDIT.