Q:

How can I include the current date in an XSL rendering?

A:

The goal is achieved by performing the following steps:

Update the XSL to map a prefix to the URL specified in the web.config file
<extension mode="on" type="Sitecore.DateUtil, Sitecore.Kernel" namespace="http://www.sitecore.net/dateutil" singleInstance="true" />
and add it to the ignored extensions, then invoke methods in that class using the prefix and method name.
Assuming the date is needed in the ISO format used by Sitecore, the extension could just return the Sitecore.DateUtil.IsoNow property.

The whole solution looks like this:

Add the following entry under /configuration/sitecore/xslExtensions in the web.config file:

Update the XSL:

xmlns:dateutil="http://www.sitecore.net/dateutil"
exclude-result-prefixes="dot sc dateutil"

Use the method:

<xsl:variable name="now" select="dateutil:get_IsoNow()" />
<xsl:value-of select="sc:formatdate( $now, 'MMM d, yyyy' )" />