What day was the surface clay and the score 6–1, 6–4?
CREATE TABLE "singles_12" (
  "date" text,
  "tournament" text,
  "surface" text,
  "opponent_in_the_final" text,
  "score" text
);
SELECT "date" FROM "singles_12" WHERE "surface"='clay' AND "score"='6–1, 6–4';