addDays method

@override Gregorian addDays(int days)

makes a new date object with added days to this date

Original date object remains unchanged

this Method is safe

Implementation

@override
Gregorian addDays(int days) {
  if (days == 0) {
    return this;
  } else {
    return Gregorian.fromJulianDayNumber(julianDayNumber + days);
  }
}