diff --git a/app/portal/__init__.py b/app/portal/__init__.py
index 81227ef..af9bae5 100644
--- a/app/portal/__init__.py
+++ b/app/portal/__init__.py
@@ -20,6 +20,13 @@ def calculate_mirror_expiry(s):
     return f"{countdown.days} days"
 
 
+@portal.app_template_filter("format_datetime")
+def format_datetime(s):
+    if s is None:
+        return "Unknown"
+    return s.strftime("%a, %d %b %Y %H:%M:%S")
+
+
 @portal.route("/")
 def portal_home():
     return render_template("home.html.j2", section="home")
diff --git a/app/portal/templates/alarms.html.j2 b/app/portal/templates/alarms.html.j2
index 362cdbe..0e8bea8 100644
--- a/app/portal/templates/alarms.html.j2
+++ b/app/portal/templates/alarms.html.j2
@@ -28,7 +28,7 @@
                     <td>{{ alarm.alarm_type }}</td>
                     <td>{{ alarm.alarm_state.name }}</td>
                     <td>{{ alarm.text }}</td>
-                    <td>{{ alarm.last_updated.strftime("%a, %d %b %Y %H:%M:%S") }}</td>
+                    <td>{{ alarm.last_updated | format_date }}</td>
                 </tr>
             {% endfor %}
             </tbody>