Returns a new DateTime instance with duration added to this.
DateTime today = new DateTime.now();
DateTime sixtyDaysFromNow = today.add(new Duration(days: 60));
Source
DateTime add(Duration duration) {
int ms = millisecondsSinceEpoch;
return new DateTime.fromMillisecondsSinceEpoch(
ms + duration.inMilliseconds, isUtc: isUtc);
}