What is Date, when Meet is "World Championships", when Nationality is "United States", and when Time is "1:46.68"?
CREATE TABLE "men" (
  "event" text,
  "time" text,
  "nationality" text,
  "date" text,
  "meet" text,
  "location" text
);
SELECT "date" FROM "men" WHERE "meet"='world championships' AND "nationality"='united states' AND "time"='1:46.68';