On how many different dates was the race at the Silverstone circuit?
CREATE TABLE "calendar" (
  "round" real,
  "circuit" text,
  "date" text,
  "pole_position" text,
  "fastest_lap" text,
  "winning_drivers" text,
  "winning_team" text
);
SELECT COUNT("date") FROM "calendar" WHERE "circuit"='Silverstone';