This page is in a suite of pages testing how search indexers do with page modification dates. If the server is dynamically setting the page date, adding Dublin Core Date tags can identify the meaningful content date.
There are explicit examples of schema Date meta tags in the HTML recommendations, and support for the Dublin Core date tags, which are standard across the Web. Again, these will probably not be sent with "HEAD" requests, but enterprise search engines can extract them during indexing for date accuracy.
When displaying dates, W3C recommends the ISO 8601 format, YYYY-MM-DD, as in the following example:
<meta name="DC.Date" content="2005-05-01">
Not all search engines will honor these tags, but they ought to!
Actual Modification Date: May 5, 2005
Search Code: RTest, RTest265, RTestGood, RTestGood265
For more tests, see the Date Test Suite and the List of Search Indexing and Robot Tests.