In HTML
<meta name="created" content="2011-01-01" />
<meta name="DC.Date.Created" scheme="ISO8601" content="2011/1/1" />or
<span id="created">2011-01-01</span>
Any date format that php strtotime() can read should be acceptable.
In code (HOOK_import_html)
$node->created = $timestamp;or
$node->date = format_date($timestamp, 'custom', 'Y-m-d\TH:i:sP'); // a string