Implement SkipLast with time.

AsyncRx.NET/System.Reactive.Async/System/Reactive/Linq/Operators/SkipLast.cs

@@ -96,7 +96,33 @@ namespace System.Reactive.Linq
             if (clock == null)
                 throw new ArgumentNullException(nameof(clock));
 
-            throw new NotImplementedException();
+            var queue = new Queue<Timestamped<TSource>>();
+
+            async Task FlushAsync(DateTimeOffset now)
+            {
+                while (queue.Count > 0 && now - queue.Peek().Timestamp >= duration)
+                {
+                    await observer.OnNextAsync(queue.Dequeue().Value).ConfigureAwait(false);
+                }
+            }
+
+            return Create<TSource>(
+                async x =>
+                {
+                    var now = clock.Now;
+
+                    queue.Enqueue(new Timestamped<TSource>(x, now));
+
+                    await FlushAsync(now).ConfigureAwait(false);
+                },
+                observer.OnErrorAsync,
+                async () =>
+                {
+                    await FlushAsync(clock.Now).ConfigureAwait(false);
+
+                    await observer.OnCompletedAsync().ConfigureAwait(false);
+                }
+            );
         }
     }
 }