code: @Override
      public HijriCalendar transform(long utcDays) {
  
          long realDays = Math.addExact(utcDays, this.adjustment);
          int monthStart = search(realDays, this.firstOfMonth);
  
          if (monthStart >= 0) {
              if (
                  (monthStart < this.firstOfMonth.length - 1)
                  || (this.firstOfMonth[monthStart] + this.lengthOfMonth[monthStart] > realDays)
              ) {
                  int hyear = (monthStart / 12) + this.minYear;
                  int hmonth = (monthStart % 12) + 1;
                  int hdom = (int) (realDays - this.firstOfMonth[monthStart] + 1);
                  return HijriCalendar.of(this.variant, hyear, hmonth, hdom);
              }
          }
  
          throw new IllegalArgumentException("Out of range: " + utcDays);
  
      }
docs: ~ Methoden ----------------------------------------------------------