My transformation included the following (which I had manually entered in JDeveloper):
<ns0:QualificationDate>
<xsl:if test="number(ns1:Highest/ns1:QualificationDate) > 0">
<xsl:value-of select="number(ns1:Highest/ns1:QualificationDate) + 190000"/>
</xsl:if>
</ns0:QualificationDate>
Turns out, the correct usage of the <if>
<ns0:QualificationDate>
<xsl:value-of select="number(ns1:Highest/ns1:QualificationDate) + 190000"/>
</ns0:QualificationDate>
</xsl:if>
It's the little things that get you sometimes.