Which date did the oakland raiders play as the opponent when the Bills first downs were under 28?
CREATE TABLE "schedule" (
  "game" real,
  "date" text,
  "opponent" text,
  "result" text,
  "bills_points" real,
  "opponents" real,
  "bills_first_downs" real,
  "record" text,
  "attendance" real
);
SELECT "date" FROM "schedule" WHERE "bills_first_downs"<28 AND "opponent"='oakland raiders';