SQL & PL/SQL

What is the different between (sysdate) + 1 and (sysdate + 1)?

SQL>BEGIN
  dbms_job.submit(JOB => jobnumber,
  WHAT => 'BEGIN INV_BACKEND.Inv_Backend; end;',
  NEXT_DATE => trunc(sysdate),
  INTERVAL => 'trunc(sysdate) + 23/24 + 1' );
COMMIT;

Is this will start the job by follow the sysdate and continue run on tomorrow 23:00?
can give me an example?
thanks