diff --git a/gateway/platforms/bluebubbles.py b/gateway/platforms/bluebubbles.py
index ecc6baf45e01..c083ee63f603 100644
--- a/gateway/platforms/bluebubbles.py
+++ b/gateway/platforms/bluebubbles.py
@@ -15,7 +15,7 @@
 import re
 import uuid
 from collections import OrderedDict
-from datetime import datetime
+from datetime import datetime, timezone
 from typing import Any, Dict, List, Optional
 from urllib.parse import quote
 
@@ -510,7 +510,7 @@ async def _create_chat_for_handle(
         payload = {
             "addresses": [address],
             "message": message,
-            "tempGuid": f"temp-{datetime.utcnow().timestamp()}",
+            "tempGuid": f"temp-{datetime.now(timezone.utc).timestamp()}",
         }
         try:
             res = await self._api_post("/api/v1/chat/new", payload)
@@ -566,7 +566,7 @@ async def send(
                 )
             payload: Dict[str, Any] = {
                 "chatGuid": guid,
-                "tempGuid": f"temp-{datetime.utcnow().timestamp()}",
+                "tempGuid": f"temp-{datetime.now(timezone.utc).timestamp()}",
                 "message": chunk,
             }
             if reply_to and self._private_api_enabled and self._helper_connected:
diff --git a/tests/gateway/test_bluebubbles.py b/tests/gateway/test_bluebubbles.py
index f4d606dca7ff..18f5aaef3c95 100644
--- a/tests/gateway/test_bluebubbles.py
+++ b/tests/gateway/test_bluebubbles.py
@@ -1,6 +1,7 @@
 """Tests for the BlueBubbles iMessage gateway adapter."""
 import asyncio
 import json
+import time
 
 import pytest
 
@@ -438,3 +439,57 @@ def raise_for_status(self):
         assert len(deleted_ids) == 2
 
 
+class TestBlueBubblesTempGuid:
+    """tempGuid must embed a UTC epoch (issue #79511).
+
+    ``datetime.utcnow()`` returns a NAIVE datetime, and ``.timestamp()`` on a
+    naive datetime interprets it as LOCAL time — so the old code produced an
+    epoch shifted by the machine's UTC offset (hours, not seconds). Both
+    outbound payload builders must use ``datetime.now(timezone.utc)`` instead.
+    """
+
+    @staticmethod
+    def _epoch_from_temp_guid(temp_guid):
+        # Format: "temp-<epoch>"
+        return float(temp_guid.split("-", 1)[1])
+
+    @staticmethod
+    def _assert_utc_epoch(temp_guid):
+        assert temp_guid.startswith("temp-")
+        # Tight tolerance (seconds) — a local-offset bug would drift by the
+        # TZ offset (hours), far beyond this window.
+        assert abs(TestBlueBubblesTempGuid._epoch_from_temp_guid(temp_guid) - time.time()) < 5
+
+    @pytest.mark.asyncio
+    async def test_create_chat_temp_guid_is_utc_epoch(self, monkeypatch):
+        adapter = _make_adapter(monkeypatch)
+        captured = {}
+
+        async def fake_api_post(path, payload):
+            captured["payload"] = payload
+            return {"data": {"guid": "msg-1"}}
+
+        monkeypatch.setattr(adapter, "_api_post", fake_api_post)
+        result = await adapter._create_chat_for_handle("+155****0100", "hello")
+        assert result.success is True
+        self._assert_utc_epoch(captured["payload"]["tempGuid"])
+
+    @pytest.mark.asyncio
+    async def test_send_temp_guid_is_utc_epoch(self, monkeypatch):
+        adapter = _make_adapter(monkeypatch)
+        captured = {}
+
+        async def fake_resolve(chat_id):
+            return "iMessage;+;chat-guid"
+
+        async def fake_api_post(path, payload):
+            captured["payload"] = payload
+            return {"data": {"guid": "msg-1"}}
+
+        monkeypatch.setattr(adapter, "_resolve_chat_guid", fake_resolve)
+        monkeypatch.setattr(adapter, "_api_post", fake_api_post)
+        result = await adapter.send("+155****0100", "hello")
+        assert result.success is True
+        self._assert_utc_epoch(captured["payload"]["tempGuid"])
+
+