When was the build date for ff20 PRR class and erie built builder's model?
CREATE TABLE "fairbanks_morse" (
  "prr_class" text,
  "builder_s_model" text,
  "build_date" text,
  "total_produced" real,
  "wheel_arrangement" text,
  "service" text,
  "power_output" text
);
SELECT "build_date" FROM "fairbanks_morse" WHERE "prr_class"='ff20' AND "builder_s_model"='erie built';