Adding a default clock.

AsyncRx.NET/System.Reactive.Async.Concurrency/System/Reactive/Concurrency/Clock.cs

@@ -0,0 +1,13 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the Apache 2.0 License.
+// See the LICENSE file in the project root for more information. 
+
+namespace System.Reactive.Concurrency
+{
+    public class Clock : IClock
+    {
+        public static Clock Default { get; } = new Clock();
+
+        public DateTimeOffset Now => DateTimeOffset.UtcNow;
+    }
+}