On how many different dates did schools from Chestnut Hill, Massachusetts join the conference?
CREATE TABLE "current_members" (
  "institution" text,
  "location" text,
  "nickname" text,
  "founded" real,
  "type" text,
  "enrollment" real,
  "joined" real
);
SELECT COUNT("joined") FROM "current_members" WHERE "location"='Chestnut Hill, Massachusetts';