diff --git a/backend/app/services/gmail_service.py b/backend/app/services/gmail_service.py
index 3fde4c7..e0726d5 100644
--- a/backend/app/services/gmail_service.py
+++ b/backend/app/services/gmail_service.py
@@ -434,6 +434,20 @@ class GmailService:
 
         return label_ids
 
+    @staticmethod
+    def _tz_aware_expiry(expiry: Optional[datetime]) -> Optional[datetime]:
+        """Return *expiry* with UTC tzinfo attached if it is naive.
+
+        google-auth sets ``credentials.expiry`` as a naive UTC datetime
+        (``datetime.utcnow() + timedelta(...)``).  Storing a naive datetime
+        into a ``DateTime(timezone=True)`` column causes silent tz-mismatch
+        bugs in comparisons and storage, so we always normalise before
+        returning expiry values to callers.
+        """
+        if expiry is not None and expiry.tzinfo is None:
+            return expiry.replace(tzinfo=timezone.utc)
+        return expiry
+
     def is_token_expiring_soon(self, within_minutes: int = 30) -> bool:
         """
         Return True if the access token has already expired or will expire
@@ -495,7 +509,7 @@ class GmailService:
             )
             return {
                 "access_token": self.credentials.token,
-                "expiry": self.credentials.expiry,
+                "expiry": self._tz_aware_expiry(self.credentials.expiry),
             }
         except google.auth.exceptions.RefreshError as e:
             error_msg = f"Gmail refresh token has been revoked or is invalid — the user must re-authorise. Detail: {e}"
@@ -529,6 +543,6 @@ class GmailService:
             )
             return {
                 "access_token": current_token,
-                "expiry": self.credentials.expiry,
+                "expiry": self._tz_aware_expiry(self.credentials.expiry),
             }
         return None
diff --git a/backend/tests/unit/test_gmail_service.py b/backend/tests/unit/test_gmail_service.py
index 31a582b..8a1160d 100644
--- a/backend/tests/unit/test_gmail_service.py
+++ b/backend/tests/unit/test_gmail_service.py
@@ -419,3 +419,72 @@ class TestGmailService:
             # Second access should NOT call build again
             _ = service.service
             mock_build.assert_called_once()
+
+    # ------------------------------------------------------------------
+    # _tz_aware_expiry – timezone normalisation helper
+    # ------------------------------------------------------------------
+    def test_tz_aware_expiry_naive_becomes_utc(self):
+        """A naive datetime is made tz-aware (UTC)."""
+        naive = datetime(2026, 6, 1, 12, 0, 0)  # no tzinfo
+        result = GmailService._tz_aware_expiry(naive)
+        assert result is not None
+        assert result.tzinfo is not None
+        assert result.utcoffset().total_seconds() == 0
+        assert result.replace(tzinfo=None) == naive
+
+    def test_tz_aware_expiry_aware_unchanged(self):
+        """An already tz-aware datetime is returned as-is."""
+        aware = datetime(2026, 6, 1, 12, 0, 0, tzinfo=timezone.utc)
+        result = GmailService._tz_aware_expiry(aware)
+        assert result is aware
+
+    def test_tz_aware_expiry_none_unchanged(self):
+        """None is returned unchanged."""
+        assert GmailService._tz_aware_expiry(None) is None
+
+    # ------------------------------------------------------------------
+    # get_refreshed_token – naive expiry is normalised to UTC
+    # ------------------------------------------------------------------
+    def test_get_refreshed_token_naive_expiry_becomes_utc(self):
+        """get_refreshed_token converts a naive expiry from google-auth to UTC-aware."""
+        service = GmailService(access_token="original-token")
+
+        # google-auth sets credentials.expiry as naive UTC
+        naive_expiry = datetime(2099, 1, 1, 0, 0, 0)  # no tzinfo
+        service.credentials.token = "new-refreshed-token"
+        service.credentials.expiry = naive_expiry
+
+        result = service.get_refreshed_token()
+        assert result is not None
+        expiry = result["expiry"]
+        assert expiry is not None
+        assert expiry.tzinfo is not None, "expiry must be tz-aware for DB storage"
+        assert expiry.utcoffset().total_seconds() == 0
+        assert expiry.replace(tzinfo=None) == naive_expiry
+
+    # ------------------------------------------------------------------
+    # proactive_refresh – naive expiry is normalised to UTC
+    # ------------------------------------------------------------------
+    @pytest.mark.asyncio
+    async def test_proactive_refresh_naive_expiry_becomes_utc(self):
+        """proactive_refresh converts a naive expiry from google-auth to UTC-aware."""
+        service = GmailService(
+            access_token="old-token",
+            refresh_token="refresh-token",
+        )
+
+        naive_expiry = datetime(2099, 6, 1, 0, 0, 0)  # no tzinfo
+
+        def _fake_refresh(_request):
+            service.credentials.token = "new-token"
+            service.credentials.expiry = naive_expiry
+
+        with patch.object(service.credentials, "refresh", side_effect=_fake_refresh):
+            result = await service.proactive_refresh()
+
+        assert result["access_token"] == "new-token"
+        expiry = result["expiry"]
+        assert expiry is not None
+        assert expiry.tzinfo is not None, "expiry must be tz-aware for DB storage"
+        assert expiry.utcoffset().total_seconds() == 0
+        assert expiry.replace(tzinfo=None) == naive_expiry