Easy. Simply re-use this former presidential pardon. Note the clause containing the words 'may have committed'

"Now, Therefore, I, [Gerald R. Ford], President of the United States, pursuant to the pardon power conferred upon me by Article II, Section 2, of the Constitution, have granted and by these presents do grant a full, free, and absolute pardon unto [Richard Nixon] for all offenses against the United States which he, [Richard Nixon], has committed or may have committed or taken part in during the period from [January 20, 1969] through [August 9, 1974]."