What to use to send an email atomatically at certain time of day

OK I have been trying to dabble for a while now and I am at the stage that if I run it myself it works fine and does what I want it, if I try to cron job it, it errors as it doesn't have all the information.

PHP Code:

public function index(){
            
            $this->load->dbutil();

            $backup = $this->dbutil->backup(); 

            $this->load->helper('file');
            $subject = 'backup-'.date("d-m-Y").' from Site Name';
            $file_name = 'backup-'.date("d-m-Y").'.gz';
            write_file($file_name, $backup); 
            $this->email->from('[email protected]', 'Backup System');
            $this->email->to('[email protected]');
            $this->email->subject($subject);
            $all = 'Filename: '.$file_name."<br>";
            $this->email->message($all);
            $this->email->attach($file_name);
            $this->email->send(); 
            delete_files('backup/');


        } 


I have removed the emails from this code.  When I get cron to run it, I get an error right at the start with the CI_controller as I know that it doesn't know what it is as its not ran everything else in the browser first.  All this is doing is grabbing the database and packing it into a file, sending it to an email address and then deleting the file in the folder (not that bothered about the deleting part atm).  Would I have to look into my own CI_controller, if so any tips on what I should be trying to make in there?