Emphasize that durations are context independent.

sdk/lib/core/date_time.dart

Index: sdk/lib/core/date_time.dart
diff --git a/sdk/lib/core/date_time.dart b/sdk/lib/core/date_time.dart
index b792ac272e6a8bfb97fdf7acfff3ddb031fe9879..0453704e1d2cfeb5915305ce59ab4a4b6da9b445 100644
--- a/sdk/lib/core/date_time.dart
+++ b/sdk/lib/core/date_time.dart
@@ -572,7 +572,14 @@ class DateTime implements Comparable<DateTime> {
    * Returns a new [DateTime] instance with [duration] added to [this].
    *
    *     DateTime today = new DateTime.now();
-   *     DateTime sixtyDaysFromNow = today.add(new Duration(days: 60));
+   *     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.
    */
   external DateTime add(Duration duration);
 
@@ -580,13 +587,14 @@ class DateTime implements Comparable<DateTime> {
    * Returns a new [DateTime] instance with [duration] subtracted from [this].
    *
    *     DateTime today = new DateTime.now();
-   *     DateTime sixtyDaysAgo = today.subtract(new Duration(days: 30));
+   *     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.
    *
-   * 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.
+   * Be careful when working with dates in local time.
    */
   external DateTime subtract(Duration duration);