Crimes Against Humanity and War Crimes Act 2000
10. Proceedings for an offence alleged to have been committed before the coming into force of this section shall be conducted in accordance with the laws of evidence and procedure in force at the time of the proceedings.
11. In proceedings for an offence under any of sections 4 to, 7, the accused may, subject to sections 12 to 14 and to subsection 607(6) of the, Criminal Code, rely on any justification, excuse or defence available under the laws of Canada or under international law at the time of the alleged offence or at the time of the proceedings .
EDIT.