addMonths method
Implementation
DateTime addMonths(int months) {
int newYear = year + (month + months - 1) ~/ 12;
int newMonth = (month + months - 1) % 12 + 1;
int newDay = day;
while (newDay > daysInMonth(newYear, newMonth)) {
newDay--;
}
return DateTime(newYear, newMonth, newDay, hour, minute, second,
millisecond, microsecond);
}