Cakephp Inline Editing

I'm working on a project for Payroll Dept. I have included inline editing feature in my code.

When the user edits start date, the program should calculate and update the hours field by subtracting the stop date from start date. I'm stuck on the calculation part giving an error "Unsupported Operand Types". Below is my code snippet

case 'event_date':
           
             if ($this->request->data['Leave']['event_date'] == $value) {
                   
                    $stop = $this->Leave->find('first', array('conditions' => array('event_date' => $value)));
                    $this->set('stop',$stop['Leave']['event_stop_time']);
                   
                    $start = $this->request->data['Leave']['event_date'];
                                   
                    $hours = ($stop - $start);
                    $this->request->data['Leave']['hours'] = $hours;
                    $this->set('updated_value', $hours);
               
             }

Any form of help is much appreciated as I'm a newbie to cakephp

Thanks

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