Calculate value with data from another model

Calculations should happen in your controller.

PHP Code:

public function myCalculation()
{
  $this->load->model('seminar');
  $this->load->model('seminar_date');
  $dataSeminar = $this->seminar->find_by_id($id);
  $dataSeminarDate = $this->seminar_date->find_by_id($id);

  // Do your calculations
}