Which date has a Score of 6–4, 6–3?
CREATE TABLE "doubles_21_13_8" (
  "outcome" text,
  "date" text,
  "tournament" text,
  "surface" text,
  "partner" text,
  "opponents_in_the_final" text,
  "score" text
);
SELECT "date" FROM "doubles_21_13_8" WHERE "score"='6–4, 6–3';