protected function getDatetimeFormat()
    {
        if( $this->interval !== '* * * * *' )
        {
            $interval = $this->interval.' ';
        }
        else
        {
            $interval = ( $this->minute    ?? '*' ) . ' '.
                        ( $this->hour      ?? '*' ) . ' '.
                        ( $this->dayNumber ?? '*' ) . ' '.
                        ( $this->month     ?? '*' ) . ' '.
                        ( $this->day       ?? '*' ) . ' ';
        }

        $this->defaultIntervalVariables();

        return $interval;
    }