round_to_interval.plpgsql

CREATE OR REPLACE FUNCTION round_to_interval (input_timestamp timestamp with time zone, interval_size interval) RETURNS timestamp with time zone AS $$   
  DECLARE
    interval_in_seconds CONSTANT int := extract('epoch' FROM interval_size);
  BEGIN
    RETURN to_timestamp(trunc(extract('epoch' FROM (input_timestamp - '1 microsecond'::interval)))::int / interval_in_seconds * interval_in_seconds + interval_in_seconds);
  END
$$
LANGUAGE plpgsql;