start_date      => <start_timestamp>,
    repeat_interval => 'freq=hourly; byminute=0',
    end_date        => <end_timestamp>,

BEGIN
 DBMS_SCHEDULER.CREATE_PROGRAM (
    program_name        => 'test_program',
    PROGRAM_TYPE        => 'STORED_PROCEDURE',
    program_action      => 'sal_percent',    
    ENABLED             => true,
    comments            => 'Program ');

    DBMS_SCHEDULER.CREATE_SCHEDULE (
    schedule_name   => 'test_schedule',
    start_date      => SYSDATE,
    repeat_interval => 'freq=minutely; byhour=18,19,20,21,22,23;interval =40',
    END_DATE        => NULL,
    comments        => 'Repeats every 40 minutes from 6pm to 11 pm');

 DBMS_SCHEDULER.CREATE_JOB (
    job_name      => 'test_job',
    program_name  => 'test_program',
    schedule_name => 'test_schedule',
    ENABLED       => TRUE,
    comments      => 'Job  ');
END;

SET SERVEROUTPUT ON;
ALTER SESSION SET NLS_DATE_FORMAT = 'DD-MON-YYYY HH24:MI:SS';
Session altered.

DECLARE
start_date        TIMESTAMP;
return_date_after TIMESTAMP;
next_run_date     TIMESTAMP;
BEGIN
start_date :=trunc(sysdate)+18/24;  
return_date_after := start_date;
FOR i IN 1..20 LOOP
  DBMS_SCHEDULER.EVALUATE_CALENDAR_STRING(  
  'freq=minutely; byhour=18,19,20,21,22,23;interval =40',
    start_date, return_date_after, next_run_date);
DBMS_OUTPUT.PUT_LINE('next_run_date: ' || next_run_date);
return_date_after := next_run_date;
END LOOP;
END;