What day did he play on hard with tom mercer?
CREATE TABLE "doubles_finals_8" (
  "date" text,
  "tournament" text,
  "surface" text,
  "partner" text,
  "opponents_in_the_final" text,
  "score" text
);
SELECT "date" FROM "doubles_finals_8" WHERE "surface"='hard' AND "partner"='tom mercer';