subtract method
Returns a new DateTime instance with duration subtracted from this.
DateTime today = new DateTime.now();
DateTime fiftyDaysAgo = today.subtract(new Duration(days: 50));
Notice that the duration being subtracted 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 earlier.
Be careful when working with dates in local time.
Implementation
external DateTime subtract(Duration duration);