diff --git a/frontend/app/javascript/dashboard/helper/specs/actionCable.spec.js b/frontend/app/javascript/dashboard/helper/specs/actionCable.spec.js
index 54753f3f..a6cd9923 100644
--- a/frontend/app/javascript/dashboard/helper/specs/actionCable.spec.js
+++ b/frontend/app/javascript/dashboard/helper/specs/actionCable.spec.js
@@ -124,7 +124,7 @@ describe('ActionCableConnector - Copilot Tests', () => {
     );
   });
 
-  it('does not apply replayed activity timestamps', async () => {
+  it('does not apply replayed or 1e10-millisecond activity timestamps', async () => {
     const currentMessage = {
       id: 42,
       conversation_id: 7,
@@ -148,7 +148,7 @@ describe('ActionCableConnector - Copilot Tests', () => {
     });
     await actionCable.onMessageCreated({
       ...currentMessage,
-      updated_at: '2026-08-24T01:00:00Z',
+      updated_at: 10_000_000_000,
       conversation: { last_activity_at: 50 },
     });
 
diff --git a/frontend/app/javascript/shared/helpers/specs/timeHelper.spec.js b/frontend/app/javascript/shared/helpers/specs/timeHelper.spec.js
index 19240a25..728da66b 100644
--- a/frontend/app/javascript/shared/helpers/specs/timeHelper.spec.js
+++ b/frontend/app/javascript/shared/helpers/specs/timeHelper.spec.js
@@ -23,12 +23,29 @@ afterEach(() => {
 
 describe('#isNewerTimestamp', () => {
   it.each([
-    ['Unix seconds', 1_700_000_000, 1_700_000_001],
-    ['numeric strings', '1700000000', '1700000001'],
-    ['early Unix milliseconds', 978_307_200_000, 1_700_000_000],
-    ['below the former 1e12 threshold', 999_999_999_999, 1_000_000_000_000],
-  ])('orders %s', (_label, current, next) => {
-    expect(isNewerTimestamp(current, next)).toBe(true);
+    ['Unix seconds', 1_700_000_000, 1_700_000_001, true],
+    ['numeric strings', '1700000000', '1700000001', true],
+    ['early Unix milliseconds', 978_307_200_000, 1_700_000_000, true],
+    [
+      'below the former 1e12 threshold',
+      999_999_999_999,
+      1_000_000_000_000,
+      true,
+    ],
+    ['below 1e10 as seconds', 9_999_999_998, 9_999_999_999, true],
+    ['1e10 as milliseconds', 9_999_999_999, 10_000_000_000, false],
+    ['above 1e10 as milliseconds', 10_000_000_000, 10_000_000_001, true],
+    ['minimum ECMAScript Date', 'invalid', -8_640_000_000_000_000, true],
+    ['below minimum Date', 'invalid', -8_640_000_000_000_001, false],
+    [
+      'maximum ECMAScript Date',
+      8_639_999_999_999_999,
+      8_640_000_000_000_000,
+      true,
+    ],
+    ['above maximum Date', 8_639_999_999_999_999, 8_640_000_000_000_001, false],
+  ])('handles %s', (_label, current, next, expected) => {
+    expect(isNewerTimestamp(current, next)).toBe(expected);
   });
 
   it('preserves missing and invalid timestamp compatibility', () => {
diff --git a/frontend/app/javascript/widget/helpers/specs/actionCable.spec.js b/frontend/app/javascript/widget/helpers/specs/actionCable.spec.js
index 5ed3cb90..3b21e363 100644
--- a/frontend/app/javascript/widget/helpers/specs/actionCable.spec.js
+++ b/frontend/app/javascript/widget/helpers/specs/actionCable.spec.js
@@ -113,7 +113,7 @@ describe('Widget ActionCableConnector', () => {
     expect(IFrameHelper.sendMessage).not.toHaveBeenCalled();
   });
 
-  it('rejects an older numeric-string timestamp against an ISO timestamp', async () => {
+  it('treats an exact 1e10 numeric string as milliseconds', async () => {
     const currentMessage = {
       id: 42,
       conversation_id: 7,
@@ -126,7 +126,7 @@ describe('Widget ActionCableConnector', () => {
     await actionCable.onMessageUpdated({
       ...currentMessage,
       content: 'older',
-      updated_at: '1787533200',
+      updated_at: '10000000000',
       previous_changes: {
         content_attributes: [{}, { submitted_values: ['yes'] }],
       },