Given a month and a year, determine the last day of the month.

<xsl:template name="date:last-day-of-month">
     <xsl:param name="month"/>
     <xsl:param name="year"/>
     <xsl:choose>
       <xsl:when test="$month = 2 and 
          not($year mod 4) and 
          ($year mod 100 or not($year mod 400))">
         <xsl:value-of select="29"/>
          </xsl:when>
       <xsl:otherwise>
         <xsl:value-of 
          select="substring('312831303130313130313031',
                         2 * $month - 1,2)"/>
       </xsl:otherwise>
     </xsl:choose>
</xsl:template>

This function has potential application for constructing pages of a calendar. It is simple enough to understand once you know the rules governing a leap year. This function was translated from Lisp, in which the number of days in each month was extracted from a list. I choose to use substring to accomplish the same task. You may prefer to place the logic in additional when elements. You might also store data about each month, including its length, in XML.

See Recipe 3.5 for ways to store calendar metadata in XML.