I believe that the usual practice is to work off of the guid element in the RSS feed. This is sometimes the URI of the source article, sometimes a number, sometimes a traditional GUID.
Using this element to see if you have already received an article will negate the need to parse for a date and this is how Google Reader usually determines if an item has already been collected.
<?xml version="1.0" encoding="UTF-8"?>
<rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom">
<channel>
<atom:link href="http://www.stevefenton.co.uk/RSS/Blog/" rel="self" type="application/rss+xml" />
<title>Steve Fenton Blog</title>
<link>http://www.stevefenton.co.uk/RSS/Blog/</link>
<description>Blog</description>
<language>en</language>
<copyright>Copyright 2008 - 2010 Steve Fenton</copyright>
<category>Blog</category>
<generator>Swift Point Content Management System</generator>
<ttl>60</ttl>
<managingEditor>[email protected] (Site Admin)</managingEditor>
<item>
<title><![CDATA[Jquery Plugin Infinite Scroller With AJAX]]></title>
<link>http://www.stevefenton.co.uk/Content/Blog/Date/201004/Blog/Jquery-Plugin-Infinite-Scroller-With-AJAX/</link>
<description><![CDATA[Friday, 9th April 2010 - Jquery Plugin Infinite Scroller With AJAX <p>I have just finished a new plugin for the jQuery framework.</p><p>The jQuery Infinite Scroller is a great way to deliver a really long list of things, in smaller chunks. For example, if you were displaying articles you could load a page with the first 10 results, then dynamically add more results to the bottom of the list when people start scrolling down. The further they scroll, the more articles you add - thus making it theoretically infinite.</p><p>When the plugin detects that no more results are available, it stops trying to get more items to add.]]> <a href="http://www.stevefenton.co.uk/Content/Blog/Date/201004/Blog/Jquery-Plugin-Infinite-Scroller-With-AJAX"&gt;View Details</a>.</description>
<guid>http://www.stevefenton.co.uk/Content/Blog/Date/201004/Blog/Jquery-Plugin-Infinite-Scroller-With-AJAX</guid>
</item>
<item>
<title><![CDATA[Auto Load Your PHP Classes]]></title>
<link>http://www.stevefenton.co.uk/Content/Blog/Date/201004/Blog/Auto-Load-Your-PHP-Classes/</link>
<description><![CDATA[Wednesday, 7th April 2010 - Auto Load Your PHP Classes <p>In PHP5 you can create classes to organise your code and represent objects that you want to pass around. This has long been a feature of other languages and was a fundamentally important step forward for PHP.</p><p>There was one thing, though, that I didn't like about PHP classes. If I wanted to instantiate a new "Customer" or "Product", I had to make sure that I included the PHP file that contained the "Customer" or "Product" class. This meant doing this:</p><p>[[#CODE:php:<br>include_once 'classes/Customer.php';</p>]]> <a href="http://www.stevefenton.co.uk/Content/Blog/Date/201004/Blog/Auto-Load-Your-PHP-Classes"&gt;View Details</a>.</description>
<guid>http://www.stevefenton.co.uk/Content/Blog/Date/201004/Blog/Auto-Load-Your-PHP-Classes</guid>
</item>
</channel>
</rss>