xslt 1.0 では、たとえば次のスタイルシートでそれを行うことができます
<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="xml" version="1.0" encoding="UTF-8" indent="yes"/>
<xsl:template match="/">
<Durations>
<xsl:apply-templates select="a/TimesheetDuration/node()[starts-with(name(),'Day')]" />
<xsl:variable name="hours">
<xsl:call-template name="sumHours">
<xsl:with-param name="Day" select="a/TimesheetDuration/node()[starts-with(name(),'Day')][1]" />
</xsl:call-template>
</xsl:variable>
<SumOfHours>
<xsl:value-of select="$hours" />
</SumOfHours>
<!-- Sum of minutes would be calculated similarly -->
</Durations>
</xsl:template>
<xsl:template match="node()[starts-with(name(),'Day')]">
<xsl:copy-of select="Duration" />
</xsl:template>
<xsl:template name="sumHours">
<xsl:param name="tmpSum" select="0" />
<xsl:param name="Day" />
<xsl:variable name="newTmpSum" select="$tmpSum + substring-before($Day/Duration, ':')" />
<xsl:choose>
<xsl:when test="$Day/following-sibling::node()[starts-with(name(),'Day')]">
<xsl:call-template name="sumHours">
<xsl:with-param name="tmpSum" select="$newTmpSum" />
<xsl:with-param name="Day" select="$Day/following-sibling::node()[starts-with(name(),'Day')]" />
</xsl:call-template>
</xsl:when>
<xsl:otherwise>
<xsl:value-of select="$newTmpSum" />
</xsl:otherwise>
</xsl:choose>
</xsl:template>
</xsl:stylesheet>
出力を生成します
<?xml version="1.0" encoding="UTF-8"?>
<Durations>
<Duration>03:00</Duration>
<Duration>04:00</Duration>
<Duration>01:00</Duration>
<SumOfHours>8</SumOfHours>
</Durations>