If there is no <date> element, the publication date for the EPUB
manifest is taken from the copyright year. But something like
"1996-2013" is not a legal date specification. So the EPUB output
currently fails epubcheck.
Put in a separate <date> element with the current year. Put it in
legal.sgml, because copyright.pl already instructs to update that
manually, so it hopefully won't be missed.
<!-- doc/src/sgml/legal.sgml -->
+<date>2013</date>
+
<copyright>
<year>1996-2013</year>
<holder>The PostgreSQL Global Development Group</holder>