zotero/translators — comment on commit d7bd0dea588c0902875025a30e7e6df13eb63d45
File: BioMed Central and More.js	 line 53
By rmzelle:

http://www.microbialcellfactories.com/content/7/1/22/ref doesn't store the date in this field. I don't know how common this is, but there are some fallback options: '//meta[@name="dc.date"/@content' and '//meta[@name="prism.publicationDate"/@content'