diff --git a/doc/src/sgml/datatype.sgml b/doc/src/sgml/datatype.sgml
index 89985ab7b16..6bc0de06988 100644
--- a/doc/src/sgml/datatype.sgml
+++ b/doc/src/sgml/datatype.sgml
@@ -1878,7 +1878,10 @@ MINUTE TO SECOND
     <para>
      Date and time input is accepted in almost any reasonable format, including
      ISO 8601, <acronym>SQL</acronym>-compatible,
-     traditional <productname>POSTGRES</productname>, and others.
+     traditional <productname>POSTGRES</productname>, and others.  (ISO
+     8601 requires years of more than four digits to be preceded by a plus
+     or minus sign;  <productname>PostgreSQL</productname> supports such
+     years, but without a sign.)
      For some formats, ordering of day, month, and year in date input is
      ambiguous and there is support for specifying the expected
      ordering of these fields.  Set the <xref linkend="guc-datestyle"/> parameter