diff --git a/elementary/monitor/alerts/alert.py b/elementary/monitor/alerts/alert.py
index eb1af224f..a31dd7a33 100644
--- a/elementary/monitor/alerts/alert.py
+++ b/elementary/monitor/alerts/alert.py
@@ -58,7 +58,11 @@ def __init__(
         self.detected_at = None
         if detected_at is not None:
             try:
-                self.detected_at_utc = detected_at
+                # detected_at is stored in UTC, so a naive datetime must be
+                # interpreted as UTC rather than as the machine's local time.
+                if detected_at.tzinfo is None:
+                    detected_at = detected_at.replace(tzinfo=tz.tzutc())
+                self.detected_at_utc = detected_at.astimezone(tz.tzutc())
                 self.detected_at = detected_at.astimezone(
                     tz.gettz(timezone) if timezone else tz.tzlocal()
                 )
diff --git a/tests/unit/monitor/alerts/test_alert_models.py b/tests/unit/monitor/alerts/test_alert_models.py
index 13c5c5673..79de2dfa6 100644
--- a/tests/unit/monitor/alerts/test_alert_models.py
+++ b/tests/unit/monitor/alerts/test_alert_models.py
@@ -1,4 +1,8 @@
+import time
+from datetime import datetime
+
 import pytest
+from dateutil import tz
 
 from elementary.monitor.alerts.alert import AlertModel
 from elementary.monitor.alerts.model_alert import ModelAlertModel
@@ -6,6 +10,15 @@
 from elementary.monitor.alerts.test_alert import TestAlertModel
 
 
+@pytest.fixture
+def tokyo_local_timezone(monkeypatch):
+    monkeypatch.setenv("TZ", "Asia/Tokyo")
+    time.tzset()
+    yield
+    monkeypatch.undo()
+    time.tzset()
+
+
 def _make_test_alert(
     test_sub_type: str = "generic", test_short_name: str = "my_test"
 ) -> TestAlertModel:
@@ -95,3 +108,75 @@ def test_asset_type_is_source(self) -> None:
     def test_concise_name_is_source_dot_identifier(self) -> None:
         alert = _make_source_freshness_alert(source_name="raw", identifier="orders")
         assert alert.concise_name == "raw.orders"
+
+
+class TestAlertModelDetectedAtTimezone:
+    """detected_at is stored in UTC (naive) in the alerts table and must be
+    interpreted as UTC, not as the machine's local time."""
+
+    def test_naive_detected_at_is_converted_to_given_timezone(
+        self, tokyo_local_timezone
+    ) -> None:
+        alert = AlertModel(
+            id="id",
+            alert_class_id="acid",
+            detected_at=datetime(2026, 7, 22, 9, 8, 22),
+            timezone="Asia/Tokyo",
+        )
+        assert alert.detected_at == datetime(
+            2026, 7, 22, 18, 8, 22, tzinfo=tz.gettz("Asia/Tokyo")
+        )
+        assert alert.detected_at_str == "2026-07-22 18:08:22 JST"
+
+    def test_naive_detected_at_is_converted_to_local_timezone_by_default(
+        self, tokyo_local_timezone
+    ) -> None:
+        alert = AlertModel(
+            id="id",
+            alert_class_id="acid",
+            detected_at=datetime(2026, 7, 22, 9, 8, 22),
+        )
+        assert alert.detected_at_str == "2026-07-22 18:08:22 JST"
+
+    def test_naive_detected_at_utc_is_utc_aware(self, tokyo_local_timezone) -> None:
+        alert = AlertModel(
+            id="id",
+            alert_class_id="acid",
+            detected_at=datetime(2026, 7, 22, 9, 8, 22),
+            timezone="Asia/Tokyo",
+        )
+        assert alert.detected_at_utc == datetime(
+            2026, 7, 22, 9, 8, 22, tzinfo=tz.tzutc()
+        )
+
+    def test_aware_detected_at_keeps_working(self, tokyo_local_timezone) -> None:
+        alert = AlertModel(
+            id="id",
+            alert_class_id="acid",
+            detected_at=datetime(2026, 7, 22, 9, 8, 22, tzinfo=tz.tzutc()),
+            timezone="Asia/Tokyo",
+        )
+        assert alert.detected_at_str == "2026-07-22 18:08:22 JST"
+
+    def test_source_freshness_result_description_uses_consistent_timezone(
+        self, tokyo_local_timezone
+    ) -> None:
+        alert = SourceFreshnessAlertModel(
+            id="id",
+            source_name="my_source",
+            identifier="my_table",
+            original_status="fail",
+            path="models/src.yml",
+            error=None,
+            alert_class_id="acid",
+            source_freshness_execution_id="sfeid",
+            detected_at=datetime(2026, 7, 22, 9, 8, 22),
+            max_loaded_at=datetime(2026, 7, 21, 8, 55, 10),
+            max_loaded_at_time_ago_in_s=87101.8,
+            timezone="Asia/Tokyo",
+        )
+        assert alert.result_description == (
+            "When the test ran at 2026-07-22 18:08:22 JST, "
+            "the most recent record found in the table was 1 day 0h 11m 41s earlier "
+            "(2026-07-21 17:55:10 JST)."
+        )