diff --git a/MainWindow.IedBrowser.cs b/MainWindow.IedBrowser.cs
index dc5c2f385..511bfcb54 100644
--- a/MainWindow.IedBrowser.cs
+++ b/MainWindow.IedBrowser.cs
@@ -584,12 +584,18 @@ private void UpdateIedBrowserControlStateDetails()
                     device.LiveValues, block, pageKind, field,
                     out var observed, out var reference))
             {
-                row.Value = observed.IsStale ? "Offline · " + observed.Value : observed.Value;
+                var display = IedBrowserControlStateEvidence.FormatObservedField(
+                    pageKind, field, observed);
+                row.Value = observed.IsStale ? "Offline · " + display : display;
                 row.ObservationSource = observed.IsStale
                     ? "Stale MMS Read" : "MMS Read";
                 row.ObservationTooltip = $"{reference} [{block.FunctionalConstraint}]" +
                     $" · {observed.Source} · {observed.ObservedUtc:yyyy-MM-dd HH:mm:ss} UTC" +
-                    $" · association {observed.Association}";
+                    $" · association {observed.Association}" +
+                    (field == "LActTm" && observed.TimeBits is
+                        { IsDecoded: true, HasWireProvenance: true } time
+                        ? $" · wire {time.RawHex} · time quality {time.QualityHex}"
+                        : "");
             }
             else
             {
diff --git a/Models/IedBrowserControlStateEvidence.cs b/Models/IedBrowserControlStateEvidence.cs
index aa4c07b76..c50073eea 100644
--- a/Models/IedBrowserControlStateEvidence.cs
+++ b/Models/IedBrowserControlStateEvidence.cs
@@ -84,6 +84,24 @@ public static bool TryRead(
         }
         return true;
     }
+    /// <summary>
+    /// SGCB LActTm is a real relay value, but an all-zero wire UTC-Time
+    /// denotes an unset time rather than an actual group-change in 1970.
+    /// A formatted date alone is never proof: require exact raw MMS bytes.
+    /// </summary>
+    public static string FormatObservedField(
+        string pageKind, string field, IedBrowserObservedValue observed)
+        => pageKind == "SGCB" && field == "LActTm" &&
+           observed.TimeBits is
+           {
+               IsDecoded: true,
+               HasWireProvenance: true,
+               SecondsSinceEpoch: 0,
+               FractionOfSecond24: 0
+           }
+           ? "Not initialized"
+           : observed.Value;
+
     private static string Canonical(string reference)
         => (reference ?? "").Trim().Replace('$', '.');
 }
diff --git a/Models/IedBrowserLiveValues.cs b/Models/IedBrowserLiveValues.cs
index 5527871ee..6bc971fe3 100644
--- a/Models/IedBrowserLiveValues.cs
+++ b/Models/IedBrowserLiveValues.cs
@@ -157,7 +157,9 @@ private static bool IsTimeReference(string key)
     {
         var fcIndex = key.LastIndexOf('|');
         return fcIndex > 2 && key.AsSpan(0, fcIndex).EndsWith(
-            ".t".AsSpan(), StringComparison.Ordinal);
+            ".t".AsSpan(), StringComparison.Ordinal) ||
+            fcIndex > 2 && key.AsSpan(0, fcIndex).EndsWith(
+                ".LActTm".AsSpan(), StringComparison.Ordinal);
     }
 
     public void PublishReport(Iec61850MonitorPoint point, bool observedReportIngress = false)
diff --git a/Services/NativeIec61850Client.cs b/Services/NativeIec61850Client.cs
index 8a18c6c42..727c9324f 100644
--- a/Services/NativeIec61850Client.cs
+++ b/Services/NativeIec61850Client.cs
@@ -1113,7 +1113,10 @@ private static string BuildDynamicDataSetName(ReportControlPlan plan)
                 raw.RawValue.ToArray(), out var verifiedQuality))
             projected.QualityEvidence = verifiedQuality;
         if ((reference.EndsWith(".t", StringComparison.Ordinal) ||
-             reference.EndsWith("$t", StringComparison.Ordinal)) &&
+             reference.EndsWith("$t", StringComparison.Ordinal) ||
+             // SGCB LActTm is the model-verified last-active-group UTC-Time.
+             reference.EndsWith(".LActTm", StringComparison.Ordinal) ||
+             reference.EndsWith("$LActTm", StringComparison.Ordinal)) &&
             raw.Kind == ArMms.MmsDataKind.UtcTime)
         {
             var evidence = ArMms.Iec61850UtcTimeEvidence.Decode(raw);
diff --git a/tests/ARSAS.Tests/M214SgLastChangedWireEvidenceTests.cs b/tests/ARSAS.Tests/M214SgLastChangedWireEvidenceTests.cs
new file mode 100644
index 000000000..7d0fafd60
--- /dev/null
+++ b/tests/ARSAS.Tests/M214SgLastChangedWireEvidenceTests.cs
@@ -0,0 +1,56 @@
+using AR.Iec61850.Mms;
+using ArIED61850Tester.Models;
+
+namespace ARSAS.Tests;
+
+public sealed class M214SgLastChangedWireEvidenceTests
+{
+    private const string Reference = "LD/LLN0.SGCB.LActTm";
+
+    [Fact]
+    public void ZeroWireUtcTime_IsUnsetGroupChange_NotA1970EngineeringEvent()
+    {
+        // Typed fixture represents a validated raw eight-byte LActTm from
+        // the pinned engine's decoder; no formatted-date guessing.
+        var time = new Iec61850UtcTimeEvidence
+        {
+            IsDecoded = true, HasWireProvenance = true,
+            RawHex = "000000000000003F",
+            SecondsSinceEpoch = 0, FractionOfSecond24 = 0,
+            Quality = 0x3F, EngineeringUtc = "1970-01-01 00:00:00 UTC"
+        };
+        Assert.True(time.HasWireProvenance);
+        Assert.Equal(0u, time.SecondsSinceEpoch);
+        var ledger = new IedBrowserLiveValues();
+        var association = ledger.BeginAssociation();
+        Assert.True(ledger.PublishForAssociation(association, Reference, "SP",
+            time.EngineeringUtc, "", "", "MMS Read", utcTimeEvidence: time));
+        Assert.True(ledger.TryGet(Reference, "SP", out var observed));
+        Assert.NotNull(observed.TimeBits);
+        Assert.Equal("Not initialized", IedBrowserControlStateEvidence.FormatObservedField(
+            "SGCB", "LActTm", observed));
+        Assert.Equal(time.RawHex, observed.TimeBits.RawHex);
+    }
+
+    [Fact]
+    public void RealTimestamp_AndUnverifiedText_AreNeverRewrittenAsUnset()
+    {
+        var nonzero = new Iec61850UtcTimeEvidence
+        {
+            IsDecoded = true, HasWireProvenance = true,
+            RawHex = "651000000000003F", SecondsSinceEpoch = 0x65100000,
+            FractionOfSecond24 = 0, Quality = 0x3F,
+            EngineeringUtc = "2023-09-24 00:00:00 UTC"
+        };
+        var value = new IedBrowserObservedValue(
+            nonzero.EngineeringUtc, "", "", "MMS Read", DateTime.UtcNow,
+            TimeBits: nonzero);
+        Assert.Equal(nonzero.EngineeringUtc,
+            IedBrowserControlStateEvidence.FormatObservedField("SGCB", "LActTm", value));
+        var legacy = value with { Value = "1970-01-01 00:00:00 UTC", TimeBits = null };
+        Assert.Equal(legacy.Value,
+            IedBrowserControlStateEvidence.FormatObservedField("SGCB", "LActTm", legacy));
+        Assert.Equal(legacy.Value,
+            IedBrowserControlStateEvidence.FormatObservedField("GoCB", "LActTm", legacy));
+    }
+}