[eschwartz]

Here is an example content.opf from a book in my library (see attachment). It appears to be using:

<meta name="calibre:timestamp" content="2013-01-20T05:45:35+00:00"/>

to correspond to calibre's "Date" field, and:

<dc:date>2003-01-02T00:00:00+00:00</dc:date>

to correspond to calibre's "Published" field.