Laravel 5.1 Modify input before form request validation

Is there a way to modify input fields inside a form request class before the validation takes place?

I want to modify some input date fields as follows but it doesn't seem to work.

When I set $this->start_dt input field to '2016-02-06 12:00:00' and $this->end_dt to '2016-02-06 13:00:00' I still get validation error end_dt must be after start_dt. Which means the input request values aren't getting changed when you update $this->start_dt and $this->end_dt inside the rules() function.

public function rules()
 {
    if ($this->start_dt){
            $this->start_dt = Carbon::createFromFormat('d M Y H:i:s', $this->start_dt . ' ' . $this->start_hr . ':'. $this->start_min . ':00');
    }

    if ($this->end_dt){
         $this->end_dt = Carbon::createFromFormat('d M Y H:i:s', $this->end_dt . ' ' . $this->end_hr . ':'. $this->end_min . ':00');
   }

   return [
    'start_dt' => 'required|date|after:yesterday',
    'end_dt' => 'required|date|after:start_dt|before:' . Carbon::parse($this->start_dt)->addDays(30)            
   ];
}

via Chebli Mohamed