Returns a new DateTime instance with duration subtracted from this.
DateTime today = new DateTime.now();
DateTime sixtyDaysAgo = today.subtract(new Duration(days: 60));
Source
DateTime subtract(Duration duration) {
int ms = millisecondsSinceEpoch;
return new DateTime.fromMillisecondsSinceEpoch(
ms - duration.inMilliseconds, isUtc: isUtc);
}