Change eventTime field to standard chrome.* API format.

chrome/common/extensions/api/experimental_notification.idl

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 [nodoc] namespace experimental.notification {
   enum TemplateType {
     // icon, title, message
     simple,
 
     // icon, title, message, expandedMessage, up to two buttons
@@ skipping 30 matching lines @@
     // Title of the notification (e.g. sender name for email).
     DOMString title;
 
     // Main notification content.
     DOMString message;
 
     // Priority ranges from -2 to 2. -2 is lowest priority. 2 is highest. Zero
     // is default.
     long? priority;
 
-    // A timestamp associated with the notification, in W3C ISO 8601 format
-    // (http://www.w3.org/TR/NOTE-datetime) or ECMAScript date time string
-    // format (http://www.ecma-international.org/ecma-262/5.1/#sec-15.9.1.15)
-    // as generated by date.toISOString(), for example "1994-11-15T12:45:26Z."
-    DOMString? eventTime;
+    // A timestamp associated with the notification, in milliseconds past the
+    // epoch (e.g. <code>Date.now() + n</code>).
+    double? eventTime;
 
     // Smaller version of the icon.
     DOMString? secondIconUrl;
 
     // Text and icons of notification action buttons.
     NotificationButton[]? buttons;
 
     // Secondary notification content.
     DOMString? expandedMessage;
 
@@ skipping 44 matching lines @@
     static void onClosed(DOMString notificationId, boolean byUser);
 
     // The user clicked in a non-button area of the notification.
     static void onClicked(DOMString notificationId);
 
     // The user pressed a button in the notification.
     static void onButtonClicked(DOMString notificationId, long buttonIndex);
   };
 
 };