The date this entry was last modified, as a standard Python 9-tuple.
Comes from
- /atom:feed/atom:entry/atom:modified
- /rss/channel/item/pubDate
- /rss/channel/item/dc:date
- /rss/channel/item/dcterms:modified
- /rdf:RDF/rdf:item/dc:date
- /rdf:RDF/rdf:item/dcterms:modified