On how many different dates was the episode written by Charlie Day aired for the first time?
CREATE TABLE "table1_29273115_1" (
  "no_in_series" real,
  "no_in_season" real,
  "title" text,
  "directed_by" text,
  "written_by" text,
  "original_air_date" text,
  "production_code" text
);
SELECT COUNT("original_air_date") FROM "table1_29273115_1" WHERE "written_by"='Charlie Day';