“The U.S. Supreme Court clarified presidential pardon power in an 1866 case (Ex Parte Garland) challenging the pardon of a former Confederate soldier by President Andrew Johnson. In its opinion, the Court stated that this power "extends to every offense known to the law, and may be exercised at any time after its commission, either before legal proceedings are taken or during their pendency, or after conviction and judgment.”

— findlaw.com