Use custom notification with timer

osu.Game/Online/Multiplayer/MultiplayerClient.cs

@@ -589,12 +589,7 @@ namespace osu.Game.Online.Multiplayer
             if (countdown == null)
                 return;
 
-            PostNotification?.Invoke(new SimpleNotification
-            {
-                Text = countdown.FinalNotification
-                    ? $"The multiplayer server is restarting in {countdown.TimeRemaining:hh\\:mm\\:ss}. This multiplayer room will be closed shortly."
-                    : $"The multiplayer server is restarting in {countdown.TimeRemaining:hh\\:mm\\:ss}."
-            });
+            PostNotification?.Invoke(new ServerShutdownNotification(countdown.TimeRemaining));
         }
 
         Task IMultiplayerClient.UserBeatmapAvailabilityChanged(int userId, BeatmapAvailability beatmapAvailability)

osu.Game/Online/Multiplayer/ServerShutdownNotification.cs

@@ -0,0 +1,49 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System;
+using Humanizer.Localisation;
+using osu.Framework.Allocation;
+using osu.Game.Overlays.Notifications;
+using osu.Game.Utils;
+
+namespace osu.Game.Online.Multiplayer
+{
+    public class ServerShutdownNotification : SimpleNotification
+    {
+        private readonly DateTimeOffset endDate;
+
+        public ServerShutdownNotification(TimeSpan duration)
+        {
+            endDate = DateTimeOffset.UtcNow + duration;
+        }
+
+        [BackgroundDependencyLoader]
+        private void load()
+        {
+            updateTime();
+        }
+
+        protected override void LoadComplete()
+        {
+            base.LoadComplete();
+            Scheduler.Add(updateTimeWithReschedule);
+        }
+
+        private void updateTimeWithReschedule()
+        {
+            updateTime();
+
+            // The remaining time on a countdown may be at a fractional portion between two seconds.
+            // We want to align certain audio/visual cues to the point at which integer seconds change.
+            // To do so, we schedule to the next whole second. Note that scheduler invocation isn't
+            // guaranteed to be accurate, so this may still occur slightly late, but even in such a case
+            // the next invocation will be roughly correct.
+            double timeToNextSecond = endDate.Subtract(DateTimeOffset.UtcNow).TotalMilliseconds % 1000;
+
+            Scheduler.AddDelayed(updateTimeWithReschedule, timeToNextSecond);
+        }
+
+        private void updateTime() => Text = $"The multiplayer server is restarting in {HumanizerUtils.Humanize(endDate.Subtract(DateTimeOffset.Now), precision: 2, minUnit: TimeUnit.Second)}.";
+    }
+}

osu.Game/Utils/HumanizerUtils.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Globalization;
 using Humanizer;
+using Humanizer.Localisation;
 
 namespace osu.Game.Utils
 {
@@ -26,5 +27,27 @@ namespace osu.Game.Utils
                 return input.Humanize(culture: new CultureInfo("en-US"));
             }
         }
+
+        /// <summary>
+        /// Turns the current or provided timespan into a human readable sentence
+        /// </summary>
+        /// <param name="input">The date to be humanized</param>
+        /// <param name="precision">The maximum number of time units to return. Defaulted is 1 which means the largest unit is returned</param>
+        /// <param name="maxUnit">The maximum unit of time to output. The default value is <see cref="TimeUnit.Week"/>. The time units <see cref="TimeUnit.Month"/> and <see cref="TimeUnit.Year"/> will give approximations for time spans bigger 30 days by calculating with 365.2425 days a year and 30.4369 days a month.</param>
+        /// <param name="minUnit">The minimum unit of time to output.</param>
+        /// <param name="toWords">Uses words instead of numbers if true. E.g. one day.</param>
+        /// <returns>distance of time in words</returns>
+        public static string Humanize(TimeSpan input, int precision = 1, TimeUnit maxUnit = TimeUnit.Week, TimeUnit minUnit = TimeUnit.Millisecond, bool toWords = false)
+        {
+            // this works around https://github.com/xamarin/xamarin-android/issues/2012 and https://github.com/Humanizr/Humanizer/issues/690#issuecomment-368536282
+            try
+            {
+                return input.Humanize(precision: precision, maxUnit: maxUnit, minUnit: minUnit);
+            }
+            catch (ArgumentException)
+            {
+                return input.Humanize(culture: new CultureInfo("en-US"), precision: precision, maxUnit: maxUnit, minUnit: minUnit);
+            }
+        }
     }
 }