diff --git a/elementary/monitor/alerts/alert.py b/elementary/monitor/alerts/alert.py
index eb1af224f..10a2aeeac 100644
--- a/elementary/monitor/alerts/alert.py
+++ b/elementary/monitor/alerts/alert.py
@@ -1,4 +1,4 @@
-from datetime import datetime
+from datetime import datetime, timezone as dt_timezone
 from typing import Dict, List, Optional
 
 from dateutil import tz
@@ -58,8 +58,16 @@ def __init__(
         self.detected_at = None
         if detected_at is not None:
             try:
-                self.detected_at_utc = detected_at
-                self.detected_at = detected_at.astimezone(
+                # Timestamps from the DB are stored as UTC but arrive as naive
+                # datetimes.  Python's astimezone() interprets a naive datetime
+                # as *local* time, so we must attach UTC tzinfo first.
+                detected_at_utc_aware = (
+                    detected_at.replace(tzinfo=dt_timezone.utc)
+                    if detected_at.tzinfo is None
+                    else detected_at
+                )
+                self.detected_at_utc = detected_at_utc_aware
+                self.detected_at = detected_at_utc_aware.astimezone(
                     tz.gettz(timezone) if timezone else tz.tzlocal()
                 )
             except Exception:
diff --git a/tests/unit/alerts/test_alert_detected_at_timezone.py b/tests/unit/alerts/test_alert_detected_at_timezone.py
new file mode 100644
index 000000000..dd510f93d
--- /dev/null
+++ b/tests/unit/alerts/test_alert_detected_at_timezone.py
@@ -0,0 +1,58 @@
+"""
+Regression tests for issue #2304:
+AlertModel must treat naive detected_at values as UTC, not local time.
+"""
+from datetime import datetime, timezone as dt_timezone
+
+import pytest
+
+from elementary.monitor.alerts.alert import AlertModel
+
+
+def _make_alert(detected_at, timezone=None):
+    return AlertModel(
+        id="test",
+        alert_class_id="cls",
+        detected_at=detected_at,
+        timezone=timezone,
+    )
+
+
+def test_naive_detected_at_treated_as_utc_not_local():
+    """A naive datetime must be interpreted as UTC, not as local wall-clock time.
+
+    Regression: alert.detected_at_utc must equal the naive input with UTC tzinfo
+    attached, regardless of the process timezone.
+    """
+    naive_utc = datetime(2026, 7, 22, 9, 8, 22)
+    alert = _make_alert(naive_utc, timezone="Asia/Tokyo")
+
+    expected_utc = naive_utc.replace(tzinfo=dt_timezone.utc)
+    assert alert.detected_at_utc == expected_utc, (
+        "detected_at_utc must be the naive input with UTC tzinfo, "
+        f"got {alert.detected_at_utc!r}"
+    )
+
+    # After conversion to JST (+09:00) the hour must be 18 (9 + 9)
+    assert alert.detected_at is not None
+    assert alert.detected_at.hour == 18, (
+        "09:08:22 UTC converted to JST (+09:00) must be 18:08:22, "
+        f"got {alert.detected_at}"
+    )
+
+
+def test_aware_detected_at_is_not_re_interpreted():
+    """An already-aware datetime must be used as-is (no double-conversion)."""
+    aware_utc = datetime(2026, 7, 22, 9, 8, 22, tzinfo=dt_timezone.utc)
+    alert = _make_alert(aware_utc, timezone="Asia/Tokyo")
+
+    assert alert.detected_at_utc == aware_utc
+    assert alert.detected_at is not None
+    assert alert.detected_at.hour == 18
+
+
+def test_none_detected_at_leaves_fields_none():
+    alert = _make_alert(None)
+    assert alert.detected_at is None
+    assert alert.detected_at_utc is None
+    assert alert.detected_at_str == "N/A"