public function nthInstallmentDate($n = 1)
 {
     $date = $this->installmentGraceDate()->copy();
     if ($this->loan->isWeeklyInstallment()) {
         $date->addWeeks($n);
     } else {
         if ($date->day == 31) {
             $date->firstOfMonth()->addMonths($n)->lastOfMonth();
         } else {
             $date->addMonths($n);
         }
     }
     return $date;
 }