diff --git a/reference/datetime/datetime.xml b/reference/datetime/datetime.xml
index df8cc18efb..e2fd667230 100644
--- a/reference/datetime/datetime.xml
+++ b/reference/datetime/datetime.xml
@@ -153,6 +153,14 @@
       <simpara>
        ISO-8601 (example: 2005-08-15T15:52:01+0000)
       </simpara>
+      <note>
+       <simpara>
+        This format is not compatible with ISO-8601, but is left this way for
+        backward compatibility reasons. Use <constant>DateTime::ATOM</constant>
+        or <constant>DATE_ATOM</constant> for compatibility with ISO-8601
+        instead.
+       </simpara>
+      </note>
      </listitem>
     </varlistentry>