Table: tvb_series_2006_3
Columns: airing_date
english_title_chinese_title
number_of_episodes
genre
official_website

Question: What is the airing date for a modern drama with more than 21 episodes?
SQL: SELECT "airing_date" FROM tvb_series_2006_3 WHERE "genre" = 'modern drama' AND "number_of_episodes" > 21