MODS has various elements for dates: dateIssued, dateCreated,
copyrightDate, and others.  After discussions both on and off of the
zotero-dev google group, I wanted to seek clarification of what was
intended to be put in each of these elements (particularly the
copyrightDate field).

MARC 260$c was for the "date of publication, distribution, etc."  It
would often be used to store the date from the copyright page of a book.
 The field could also include a copyright date, often prefixed by a "c".
 I suppose that the copyright page of a book may differentiate two
separate dates or the copyright registration records could be consulted
for that.  A lot of MODS records that have been converted from MARC
therefore put 260$c into dateIssued unless the date is prefixed by a
"c", in which case they use "copyrightDate".  But it is not clear to me
that modern data should embrace these legacy conventions.

One example is Bruce D'Arcus's book "Boundaries of dissent : protest &
state power in the media age." gives the following dates:
		Registration Date:	2005-12-02
		Date of Creation:	2005
		Date of Publication:	2005-11-18 gives what is presumably a date of distribution:
	although other sellers give the date as:
	the copyright page at the front of the book claims:

The last of these is what is traditionally cited in a bibliography, I
think.  Traditionally, it has certainly been the easiest date for
readers to find.  The popularity of online book sellers has made the the
distribution date quite easy for readers to find as well.  I would
therefore expect reference management software to give preference to
these two bits of information over the more mundane details of copyright
registration.  An export may look like:

  <dateIssued encoding="w3cdtf">2005-11</dateIssued>
  <copyrightDate encoding="w3cdtf" keyDate="yes">2006</copyrightDate>

dateIssued is "the date that the resource was published, released or
issued."  Comparing cocatalog & amazon, we see that these dates can
actually differ.  In this example, they're close enough.

copyrightDate is "a date in which a resource is copyrighted."  In Berne
convention countries, the copyright date is the date of creation.  The
copyright registration may be a different date (it is encouraged to be
done within five years of publication in the US to be prima facie
evidence), but the MODS documentation does note specify that this is
intended to be a registration date & this seems more tedious for
catalogers and citation managers to find.  I have chosen the date
provided in the copyright notice because of my bias for using this in
reference management software.  I don't know if this bias is that
different from other catalogers, who will key info from the physical
monograph, though.

As Bruce said "MODS is maybe a little too clever on dates for its own
good."  However, I thought I'd make sure that we're not missing anything
& are encoding this as well as possible, given the constraints.

--Richard Karnesky