Implement TakeLastBuffer with time.

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

@@ -110,7 +110,45 @@ namespace System.Reactive.Linq
             if (clock == null)
                 throw new ArgumentNullException(nameof(clock));
 
-            throw new NotImplementedException();
+            var queue = new Queue<Timestamped<TSource>>();
+
+            void Trim(DateTimeOffset now)
+            {
+                while (queue.Count > 0 && now - queue.Peek().Timestamp >= duration)
+                {
+                    queue.Dequeue();
+                }
+            }
+
+            return Create<TSource>(
+                x =>
+                {
+                    var now = clock.Now;
+
+                    queue.Enqueue(new Timestamped<TSource>(x, now));
+
+                    Trim(now);
+
+                    return Task.CompletedTask;
+                },
+                observer.OnErrorAsync,
+                async () =>
+                {
+                    Trim(clock.Now);
+
+                    var n = queue.Count;
+
+                    var res = new List<TSource>(n);
+
+                    while (queue.Count > 0)
+                    {
+                        res.Add(queue.Dequeue().Value);
+                    }
+
+                    await observer.OnNextAsync(res).ConfigureAwait(false);
+                    await observer.OnCompletedAsync().ConfigureAwait(false);
+                }
+            );
         }
     }
 }