DateTime add(
Duration duration
)

Returns a new DateTime instance with duration added to this.

DateTime today = new DateTime.now();
DateTime sixtyDaysFromNow = today.add(new Duration(days: 60));

Source

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