We have also encountered this problem generating files for Crossref and I thought it worth documenting. I generated a test XML file and ran it through their XML validator, it complained that there were no publications dates at a journal article level. However, the Crossref documentation states that this element is optional (see page 99 here http://www.crossref.org/schema/info/Cro ... 04.1.0.pdf
). I realise the documentation is for 4.1.0 rather than 4.3.0 but its the best I could find. In addition my reading of the XSD is that it is meant to be optional :-
<xsd:element ref="titles" minOccurs="0"/>
<xsd:element ref="contributors" minOccurs="0"/>
<xsd:element ref="publication_date" minOccurs="0"/>
<xsd:element ref="description" minOccurs="0"/>
<xsd:element ref="format" minOccurs="0"/>
So I don't know why the validator insists on it being present, I suspect the XSD is not quite right. I'll report this to Crossref but in the meantime I guess we have to investigate generating publication dates at article level.