I adjusted my when condition to this:

<xsl:when test="//xs:element[ @name = local-name(current()) ]/@type = 'xs:dateTime'">
  <xsl:choose>
    <xsl:when test=". = '1899-12-30T00:00:00-04:00'">
      <Data ss:Type="String">
      </Data>
    </xsl:when>
    <xsl:otherwise>
      <Data ss:Type="String">
        <xsl:value-of select="."/>
      </Data>
    </xsl:otherwise>
  </xsl:choose>
</xsl:when>

This resolves this issue. If there is something better, don't hesitate to let me know.