On what date was the winning score –20 (68-67-65-64=264)?
CREATE TABLE "pga_tour_wins_13" (
  "date" text,
  "tournament" text,
  "winning_score" text,
  "margin_of_victory" text,
  "runner_s_up" text
);
SELECT "date" FROM "pga_tour_wins_13" WHERE "winning_score"='–20 (68-67-65-64=264)';