Welcome to the project.  I started looking at RSS a while back, and found loads of examples of xsl stylesheets, though it was more than a frustrating experience.  Now, for your problem at hand, yes, I'd start with modifying the existing xsl included, although, I'd recommend creating a new version and saving it, giving it a descriptive name.  Something like - rssJustpubDate.xsl.  Then, take your newsfeed module and use the new xsl to transform it.  For example, the default xsl does not include the pubDate element, because it is NOT part of the RSSv0.91 spec, so all you need to do is take out the pieces you don't want and put in the ones you want.  Of course, you must have a newsfeed that includes the pubDate, too!

Default DNN91.xsl

<?xml version="1.0"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="html" indent="yes"/>
<xsl:param name="TITLE"/><xsl:template match="rss">
  <!-- Do not show channel image -->
  <xsl:for-each select="channel/item">
    <br>
    <strong><a href="{link}" target="_main"><xsl:value-of select="title"/></a></strong><br></br>
    <!-- only display markup for description if it's present -->
    <xsl:value-of select="description"/>
    </br>
    <br></br>
  </xsl:for-each>
</xsl:template>
<xsl:template match="description">
  <br>
    <xsl:value-of select="."/>
  </br>
</xsl:template>
</xsl:stylesheet>

Modified rssJustpubDate.xsl

<?xml version="1.0"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="html" indent="yes"/>
<xsl:param name="TITLE"/>
<xsl:template match="rss">
  <xsl:for-each select="channel/item">
  <div>  
    <strong><a href="{link}" target="_main"><xsl:value-of select="title"/></a></strong><br/>
      <xsl:value-of select="pubDate"/>
    </div>
 </xsl:for-each>
</xsl:template>
</xsl:stylesheet>

I used the above stylesheet with a Newsfeed module, using the RSS from these forums as a source.  It renders the Title (link) and the pubdate on the next line. This should give you a place to start...

News Feeds (RSS)

Install problem Security Issue??? Sat, 03 Sep 2005 13:49:14 GMT Firs time Install of DNN 3.01.01 HELP HELP HELP Thu, 08 Sep 2005 04:49:15 GMT Is there any forum module pre-installed with DNN? Fri, 09 Sep 2005 13:14:08 GMT Fresh Install Issue Fri, 09 Sep 2005 11:41:36 GMT ERRORS with fresh dnn 3.1.1 install Fri, 09 Sep 2005 17:24:41 GMT

Ok, now that you've seen an example of how to display the elements you want, Title, and pubDate, how can you change the format of the pubDate to show only the date?  That's where learning more about xsl comes in to play.  There's no substitute for learning about specific functions from any one of the fine XML resources, you can uncover with a google search.  However, I'll show you 1 way to do it, and leave the rest for you to explore.

Here's a source RSS newsfeed:

<?xml version="1.0" encoding="iso-8859-1"?>
<rss version="0.91">
  <channel>
     <title>HandCoded News</title>
     <link>http://www.hgworks.com/DesktopDefault.aspx?tabid=25</link>
     <description>HGWorks ...bringing the vision into focus.  Link for more info.</description>
     <language>en-us</language>
     <copyright>Copyright 2003-2004 HGWorks / Handcoded</copyright>
     <managingEditor>[email protected]</managingEditor>
     <webMaster>[email protected]</webMaster>
   <pubDate>Sat, 12 Mar 2005 20:21 GMT</pubDate>
   <lastBuildDate>Sat, 12 Mar 2005 20:21 GMT</lastBuildDate>
  <image>
 <title>Our Support Team</title>
 <url>http://www.hgworks.com/images/xml.gif</url>
 <link>http://hgworks.com/Portals/6b6e1a80-f39d-4ee0-b692-2d81ed9cfb26/hghcNews.xml</link>
 <width>32</width>
 <height>14</height>
 <description>Subscribe to our news feed by clicking this link button.  Copy the URL and insert into your favorite news feed reader.
(&#169; 2004 - www.hgworks.com/handcoded)</description>
  </image>
<item>
 <title>Nukeleus - DNN Installer Beta</title>
      <pubDate>Sat, 12 Mar 2005 20:21 GMT</pubDate>
      <link>http://www.asp.net/Forums/ShowPost.aspx?tabindex=1&amp;PostID=853647</link>
      <description>Check out this utility for installing DNN.  It's in beta, but it's going to make a lot of folks happy...
      </description>
</item>
<item>
 <title>DotNetNuke v3.x Released</title>
      <pubDate>Sat, 12 Mar 2005 20:21 GMT</pubDate>
      <link>http://www.asp.net/Forums/ShowPost.aspx?tabindex=1&amp;PostID=855467</link>
      <description>DNNv3.x released. Check the official announcement for details...
      </description>
</item>
   </channel>
</rss>

And a sample xsl that transforms the feed using an xsl function to extract just the date portion of the item element's pubDate.  Just a matter of using the substring function, which is handy for limiting the number of characters to display for any string.  I mostly use it to limit the number of characters in a description element when I need to do that chore.  However, it works pretty cool for this solution, too.

<?xml version="1.0"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="html" indent="yes"/>
<xsl:param name="TITLE"/>
<xsl:template match="rss">
  <xsl:for-each select="channel/item">
  <div>  
    <strong><a href="{link}" target="_main"><xsl:value-of select="title"/></a></strong><br/>
      <xsl:value-of select="substring(pubDate,1,16)"/>
    </div>
 </xsl:for-each>
</xsl:template>
</xsl:stylesheet>

The output looks like this:

Nukeleus - DNN Installer Beta
Sat, 12 Mar 2005

Yes, that's a great link for info.  The functions should be useful, but the main thing is to test - test - test.  Browser implementation of xml support varies, and I find testing the xsl very beneficial.  Now, I illustrated methods from the standpoint of transforming the xml/rss feed element for pubDate, but there are ways to affect the actual creation of the rss feed, which is a core function.  I generally do not advocate making changes to Core functions, unless you absolutely have to do it, and then, you have to really be careful to plan the change, and keep track of the changes, so the next upgrade doesn't blow your work and site away