When did the episode seen by  is 2.267 million people get aired?
CREATE TABLE "table1_21055055_2" (
  "num" real,
  "episode" text,
  "au_air_date" text,
  "timeslot" text,
  "viewers_m" text,
  "nightly_rank_num" real,
  "weekly_rank_num" real
);
SELECT "au_air_date" FROM "table1_21055055_2" WHERE "viewers_m"='2.267';