code: public function diff(AstroDate $b)
      {
          $prec = 12;
          $jd1  = $this->toJD($prec);
          $jd2  = $b->toJD($prec);
          $days = bcsub($jd1, $jd2, $prec);
  
          return Time::days(-1 * $days);
      }
docs: Finds the difference between this AstroDate and another
  
  @param  AstroDate $b
  
  @return static