What is the airing date for a modern drama with more than 21 episodes?
CREATE TABLE "second_line_series" (
  "airing_date" text,
  "english_title_chinese_title" text,
  "number_of_episodes" real,
  "genre" text,
  "official_website" text
);
SELECT "airing_date" FROM "second_line_series" WHERE "genre"='modern drama' AND "number_of_episodes">21;