So you set a parameter on the transformation and catch it in the transformation. If the date parameter was not sent to the transformation it will default to an empty string, ''.

<xsl:stylesheet ...attrs...>
 <xsl:param name="current-date" select="''"/>

 <xsl:template match="/">
   <xsl:choose>
     <xsl:when test="not($current-date='')">
       <xsl:value-of select=""/>
     </xsl:when>
     <xsl:otherwise>
       <xsl:text>No current date was passed in...</xsl:text>
     </xsl:otherwise>
 </xsl:template>

good luck,
-Rob