Returns a new DateTime instance with duration added to this.
DateTime today = new DateTime.now();
DateTime fiftyDaysFromNow = today.add(new Duration(days: 50));
Notice that the duration being added is actually 50 * 24 * 60 * 60
seconds. If the resulting DateTime has a different daylight saving offset
than this, then the result won't have the same time-of-day as this, and
may not even hit the calendar date 50 days later.
Be careful when working with dates in local time.
Source
external DateTime add(Duration duration);