Name the number of year for june 30
CREATE TABLE "table1_23015396_1" (
  "year" real,
  "date" text,
  "driver" text,
  "manufacturer" text,
  "laps" real,
  "miles_km" text,
  "race_time" text,
  "average_speed_mph" text
);
SELECT COUNT("year") FROM "table1_23015396_1" WHERE "date"='June 30';