%pub-date.class;
Publication Date Class Elements
The element <pub-date> and a <date-not-available> element. The later is used as an alternative to <pub-date>; it is an empty element flag meaning “we do not know the publication date”, which does not mean “unpublished”, necessarily, just unknown.
Model
<!ENTITY % pub-date.class
"date-not-available | pub-date" >