Reapply "Add support for timezone offset and timezone name."

corelib/src/date.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // Dart core library.
 
 /**
  * Date is the public interface to a point in time.
  */
 interface Date extends Comparable, Hashable default DateImplementation {
@@ skipping 95 matching lines @@
   /**
    * Returns a new [Date] in the given [targetTimeZone] time zone. The
    * [value] of the new instance is equal to [:this.value:].
    *
    * This call is equivalent to
    *  [:new Date.fromEpoch(this.value, targetTimeZone):].
    */
   Date changeTimeZone(TimeZone targetTimeZone);
 
   /**
+   * Returns the abbreviated time-zone name.
+   *
+   * Examples: [:"CET":] or [:"CEST":].
+   */
+  String get timeZoneName();
+
+  /**
+   * The time-zone offset is the difference between local time and UTC. That is,
+   * the offset is positive for time zones west of UTC.
+   *
+   * Note, that JavaScript, Python and C return the difference between UTC and
+   * local time. Java, C# and Ruby return the difference between local time and
+   * UTC.
+   */
+  Duration get timeZoneOffset();
+
+  /**
    * Returns the year.
    */
   int get year();
 
   /**
    * Returns the month in the year [1..12].
    */
   int get month();
 
   /**
@@ skipping 63 matching lines @@
   /**
    * Returns a new [Date] with the [duration] subtracted from this instance.
    */
   Date subtract(Duration duration);
 
   /**
    * Returns a [Duration] with the difference of [:this:] and [other].
    */
   Duration difference(Date other);
 }