We can create a pipeline 'tideinfo.pd' that uses the 'gettide.pd' pipeline:

<p:pipe xmlns:p="urn:publicid:IDN+smallx.com:pipeline:1.0" name="tideinfo"
   xmlns:c="urn:publicid:IDN+smallx.com:component-language:1.0"
   xmlns:f="urn:publicid:IDN+smallx.com:server:forms:post:1.0"
   xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
   xmlns:h="http://www.w3.org/1999/xhtml"
   xmlns:t="http://www.smallx.com/services/tideinfo/2005"
   xmlns:date="http://exslt.org/dates-and-times"
>
  <p:subtree select="t:tideinfo">
    <p:pipeline src="gettide.pd"/>
  </p:subtree>
</p:pipe>

Let's add today's date by adding this step before the [p:]pipeline step.

<p:template>
  <xsl:for-each select="t:tideinfo">
  <xsl:copy>
    <xsl:attribute name="today"><xsl:value-of select="date:date()"/></xsl:attribute>
    <xsl:copy-of select="@*|node()"/>
  </xsl:copy>
  </xsl:for-each>
</p:template>