Returns a new DateTime instance with duration subtracted from this.
DateTime today = new DateTime.now();
DateTime sixtyDaysAgo = today.subtract(new Duration(days: 30));
Notice that duration being subtracted is actually 30 * 24 * 60 * 60 seconds
and if that crosses a daylight saving time change, the resulting DateTime
won't have the same time of day as today, and may not actually hit the
calendar date 30 days earlier. Be careful when working with dates in local
time.
Source
external DateTime subtract(Duration duration);