addMonths method
Returns the date - DateTime.month + months with the DateTime.day
as the dueDay, clamped to the months length.
Implementation
@override
DateTime addMonths(DateTime date, int months) {
if (months == 0) return date;
if (months.isNegative) return addMonths(previous(date), months + 1);
return addMonths(next(date), months - 1);
}