Note that Microdata is specified for HTML5, not for HTML 4.01 (related question). If you don’t care about that:
HTML 4.01 doesn’t define a dateCreated attribute for the span element. There is also no time or data element defined. Some would misuse the abbr element, but I wouldn’t recommend that. So you’d probably have to use a span element, and, ugly, hide the machine-readable date for your visitors.
<span><span itemprop="datePublished" class="hidden">2012-01-07T07:07:21+00:00</span> January 7, 2012</span>
But better don’t use Microdata in HTML 4.01 documents in the first place. You could use RDFa instead (you can also use the Schema.org vocabulary in RDFa).