What is the date for the grass surface final against Nathan Healey?
CREATE TABLE "futures_titles" (
  "date" text,
  "tournament" text,
  "surface" text,
  "opponent_in_the_final" text,
  "score" text
);
SELECT "date" FROM "futures_titles" WHERE "surface"='grass' AND "opponent_in_the_final"='nathan healey';