I'm following this web article on configuring content query web part - http://erikswenson.blogspot.com/2010/03/sharepoint-2010-content-query-for-blog.html
When I try to add this line to my xsl
<xsl:variable name="StartDate">
<xsl:value-of select="ddwrt:FormatDateTime(string(@Modified), 1033, 'g')" />
</xsl:variable>
I get an error message
Unable to display this Web Part. To troubleshoot the problem, open this Web page in a Microsoft SharePoint Foundation-compatible HTML editor such as Microsoft SharePoint Designer. If the problem persists, contact your Web server administrator.
Correlation ID:e684f7ba-32f3-418b-83a9-e2c4f228d5e5
The log file has this.
Error while executing web part: System.Xml.Xsl.XslLoadException: Prefix 'ddwrt' is not defined
Where/How do I add this namespace to ItemStyle.xsl?