+-- |'W3CDateTime' represents a W3C Date and Time format.
+--
+-- The field 'w3cYear' is mandatory while other fields are
+-- optional. But you should be careful about combinations of such
+-- optional fields. No combinations are allowed except for the
+-- following list:
+--
+-- * YYYY
+--
+-- * YYYY-MM
+--
+-- * YYYY-MM-DD
+--
+-- * YYYY-MM-DDThh:mmTZD
+--
+-- * YYYY-MM-DDThh:mm:ss.sTZD
+--