diff --git a/apps/emqx_retainer/src/emqx_retainer_api.erl b/apps/emqx_retainer/src/emqx_retainer_api.erl
index 313ae9b02..503827d4b 100644
--- a/apps/emqx_retainer/src/emqx_retainer_api.erl
+++ b/apps/emqx_retainer/src/emqx_retainer_api.erl
@@ -46,9 +46,9 @@ message_props() ->
         {topic, string, <<"MQTT Topic">>},
         {qos, string, <<"MQTT QoS">>},
         {payload, string, <<"MQTT Payload">>},
-        {publish_at, string, <<"publish datetime">>},
-        {from_clientid, string, <<"publisher ClientId">>},
-        {from_username, string, <<"publisher Username">>}
+        {publish_at, string, <<"Publish datetime, in RFC 3339 format">>},
+        {from_clientid, string, <<"Publisher ClientId">>},
+        {from_username, string, <<"Publisher Username">>}
     ]).
 
 parameters() ->
@@ -170,7 +170,7 @@ format_message(#message{id = ID, qos = Qos, topic = Topic, from = From, timestam
     #{msgid => emqx_guid:to_hexstr(ID),
       qos => Qos,
       topic => Topic,
-      publish_at => erlang:list_to_binary(emqx_mgmt_util:strftime(Timestamp div 1000)),
+      publish_at => list_to_binary(calendar:system_time_to_rfc3339(Timestamp, [{unit, millisecond}])),
       from_clientid => to_bin_string(From),
       from_username => maps:get(username, Headers, <<>>)
      }.