dart:core

add method

DateTime add(Duration duration)

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);