diff --git a/IoListTestingWindow.ClockSyncUx.cs b/IoListTestingWindow.ClockSyncUx.cs
new file mode 100644
index 00000000..17486929
--- /dev/null
+++ b/IoListTestingWindow.ClockSyncUx.cs
@@ -0,0 +1,89 @@
+using System.Windows;
+using System.Windows.Controls;
+using System.Windows.Media;
+
+namespace ArIED61850Tester;
+
+public partial class IoListTestingWindow
+{
+    private CheckBox? _clockSyncCheckBox;
+    private bool _clockSyncCheckBoxRefreshing;
+
+    protected override void OnInitialized(EventArgs e)
+    {
+        base.OnInitialized(e);
+        Loaded += ClockSyncUx_Loaded;
+    }
+
+    private void ClockSyncUx_Loaded(object sender, RoutedEventArgs e)
+    {
+        if (_clockSyncCheckBox != null)
+        {
+            RefreshClockSyncCheckBox();
+            return;
+        }
+
+        if (WorkspacePreviewToggle.Parent is not Panel actionPanel)
+            return;
+
+        var previewIndex = actionPanel.Children.IndexOf(WorkspacePreviewToggle);
+        if (previewIndex < 0)
+            return;
+
+        var checkBox = new CheckBox
+        {
+            Name = "ClockSyncEnabledCheckBox",
+            Content = "Clock Sync",
+            VerticalAlignment = VerticalAlignment.Center,
+            Margin = new Thickness(0, 0, 12, 0),
+            Padding = new Thickness(2, 0, 2, 0),
+            FontSize = 11.2,
+            FontWeight = FontWeights.SemiBold,
+            Foreground = TryFindResource("Ink") as Brush ?? Brushes.DimGray,
+            Focusable = false,
+            ToolTip = "SNTP laptop → IED. Checked: ARSAS serves and broadcasts laptop time using the SIPROTEC compatibility profile (stratum 2). Unchecked: ARSAS stops its SNTP service. IEC 61850 monitoring is unaffected."
+        };
+
+        _clockSyncCheckBox = checkBox;
+        RefreshClockSyncCheckBox();
+        checkBox.Checked += ClockSyncCheckBox_Changed;
+        checkBox.Unchecked += ClockSyncCheckBox_Changed;
+        actionPanel.Children.Insert(previewIndex + 1, checkBox);
+    }
+
+    private void RefreshClockSyncCheckBox()
+    {
+        if (_clockSyncCheckBox == null || Owner is not MainWindow mainWindow)
+            return;
+
+        _clockSyncCheckBoxRefreshing = true;
+        try
+        {
+            _clockSyncCheckBox.IsChecked = mainWindow.IsClockSyncEnabled;
+        }
+        finally
+        {
+            _clockSyncCheckBoxRefreshing = false;
+        }
+    }
+
+    private async void ClockSyncCheckBox_Changed(object sender, RoutedEventArgs e)
+    {
+        if (_clockSyncCheckBoxRefreshing ||
+            _clockSyncCheckBox == null ||
+            Owner is not MainWindow mainWindow)
+            return;
+
+        var requested = _clockSyncCheckBox.IsChecked == true;
+        _clockSyncCheckBox.IsEnabled = false;
+        try
+        {
+            await mainWindow.SetClockSyncEnabledAsync(requested);
+            RefreshClockSyncCheckBox();
+        }
+        finally
+        {
+            _clockSyncCheckBox.IsEnabled = true;
+        }
+    }
+}
diff --git a/MainWindow.ClockSyncToggle.cs b/MainWindow.ClockSyncToggle.cs
new file mode 100644
index 00000000..b6a22226
--- /dev/null
+++ b/MainWindow.ClockSyncToggle.cs
@@ -0,0 +1,53 @@
+using ArIED61850Tester.Models;
+
+namespace ArIED61850Tester;
+
+public partial class MainWindow
+{
+    private bool _clockSyncEnabled = true;
+
+    internal bool IsClockSyncEnabled => _clockSyncEnabled;
+
+    internal async Task SetClockSyncEnabledAsync(bool enabled)
+    {
+        if (_clockSyncEnabled == enabled)
+            return;
+
+        _clockSyncEnabled = enabled;
+
+        if (!enabled)
+        {
+            Devices.CollectionChanged -= ClockSyncDevices_CollectionChanged;
+            foreach (var device in Devices)
+                device.PropertyChanged -= ClockSyncDevice_PropertyChanged;
+
+            await _clockSyncIntegrationGate.WaitAsync();
+            try
+            {
+                await _sntpClockService.StopAsync();
+                _clockSyncObservedClients.Clear();
+                AddLog("INFO", "Clock Sync",
+                    "Clock Sync disabled from the FAT workspace. IEC 61850 monitoring remains active.");
+            }
+            catch (Exception ex)
+            {
+                AddLog("WARN", "Clock Sync",
+                    $"Clock Sync disable requested, but the SNTP service reported: {ex.Message}");
+            }
+            finally
+            {
+                _clockSyncIntegrationGate.Release();
+            }
+
+            return;
+        }
+
+        Devices.CollectionChanged -= ClockSyncDevices_CollectionChanged;
+        Devices.CollectionChanged += ClockSyncDevices_CollectionChanged;
+        foreach (var device in Devices)
+            AttachClockSyncDevice(device);
+
+        AddLog("INFO", "Clock Sync",
+            "Clock Sync enabled from the FAT workspace. ARSAS will advertise laptop time to connected IPv4 IEDs using the SIPROTEC-compatible SNTP profile.");
+    }
+}
diff --git a/MainWindow.IoTesting.cs b/MainWindow.IoTesting.cs
index 0a16cb8d..c81c9d51 100644
--- a/MainWindow.IoTesting.cs
+++ b/MainWindow.IoTesting.cs
@@ -387,7 +387,7 @@ private IoTestSessionController CreateIoTestSession(IoTestProject project, strin
         => new(
             project,
             ResolveIoTestDevice,
-            action => Dispatcher.BeginInvoke(action, DispatcherPriority.Background),
+            action => Dispatcher.BeginInvoke(action, DispatcherPriority.DataBind),
             evidenceRoot);
 
     private static string IoTestingProjectsRoot() => Path.Combine(
diff --git a/tests/ARSAS.Tests/IoFatImmediateEvidenceRegressionTests.cs b/tests/ARSAS.Tests/IoFatImmediateEvidenceRegressionTests.cs
new file mode 100644
index 00000000..f0a39b2a
--- /dev/null
+++ b/tests/ARSAS.Tests/IoFatImmediateEvidenceRegressionTests.cs
@@ -0,0 +1,64 @@
+using System.ComponentModel;
+using ArIED61850Tester.Models.IoTesting;
+using ArIED61850Tester.Services.IoTesting;
+
+namespace ARSAS.Tests;
+
+public sealed class IoFatImmediateEvidenceRegressionTests
+{
+    [Fact]
+    public void TrueEdge_PublishesOnTimestampBeforeFalseEdgeArrives()
+    {
+        var evaluator = new IoTestTransitionEvaluator();
+        var point = CreatePoint();
+        var onTimestampNotificationRaised = false;
+
+        point.Runtime.PropertyChanged += (_, args) =>
+        {
+            if (args.PropertyName == nameof(IoTestPointRuntime.OnRelayTimestampText))
+                onTimestampNotificationRaised = true;
+        };
+
+        evaluator.StartAttempt(point, Observation(false, 1));
+        var on = evaluator.Observe(point, Observation(true, 2));
+
+        Assert.Equal(IoTestPointState.OnCaptured, on.State);
+        Assert.NotNull(point.Runtime.OnEvidence);
+        Assert.Null(point.Runtime.OffEvidence);
+        Assert.Equal(
+            point.Runtime.OnEvidence!.IedTimestamp?.ToString("yyyy-MM-dd HH:mm:ss.fff"),
+            point.Runtime.OnRelayTimestampText);
+        Assert.NotEqual("—", point.Runtime.OnRelayTimestampText);
+        Assert.True(onTimestampNotificationRaised);
+    }
+
+    private static IoTestPointPlan CreatePoint()
+        => new()
+        {
+            TestPointId = "TRUE-EVIDENCE-REGRESSION",
+            IedName = "SIPROTEC",
+            IpAddress = "192.168.1.10",
+            SignalName = "Protection pickup",
+            ObjectReference = "SIPROTEC/PROT.Op.general",
+            FunctionalConstraint = "ST",
+            ExpectedOnText = "true",
+            ExpectedOffText = "false",
+            ImportReady = true,
+            BindingStatus = "CID_DATASET_EXACT"
+        };
+
+    private static IoTestObservation Observation(bool state, long sequence)
+    {
+        var captured = new DateTimeOffset(2026, 8, 13, 4, 55, 0, TimeSpan.Zero)
+            .AddMilliseconds(sequence * 100);
+        return new IoTestObservation(
+            state,
+            state ? "true" : "false",
+            captured,
+            captured.AddMilliseconds(-4),
+            "Good",
+            "BRCB",
+            sequence,
+            1);
+    }
+}