[Core] Add 'now' service

Add a service to wrap Date.now(), such that this can be injected
(simplifying testing of scripts which need to respond to system
time.) Specifically supports staleness indication, WTD-660.

platform/core/src/services/Now.js

@@ -0,0 +1,27 @@
+/*global define*/
+
+define(
+    [],
+    function () {
+        "use strict";
+
+        /**
+         * Defines the `now` service, which is a simple wrapper upon
+         * `Date.now()` which can be injected to support testability.
+         *
+         * @returns {Function} a function which returns current system time
+         */
+        function Now() {
+            /**
+             * Get the current time.
+             * @returns {number} current time, in milliseconds since
+             *          1970-01-01 00:00:00Z
+             */
+            return function () {
+                return Date.now();
+            };
+        }
+
+        return Now;
+    }
+);