public function store_or_edit_shift($request, $shift_id = NULL)
 {
     $shift = $this->input->post('shift');
     $description = $this->input->post('description');
     $start_time = date("H:i", strtotime($this->input->post('start_time')));
     $end_time = date("H:i", strtotime($this->input->post('end_time')));
     $data = array('name' => $shift, 'description' => $description, 'start_time' => $start_time, 'end_time' => $end_time);
     if ($request == 'add') {
         return Shifts_model::insert_shift($data);
     } else {
         return Shifts_model::update_shift($shift_id, $data);
     }
 }