Set date/time value of child element having *prop_name* to *value*.

def _set_element_datetime(self, prop_name, value):
        """
        Set date/time value of child element having *prop_name* to *value*.
        """
        if not isinstance(value, datetime):
            tmpl = (
                "property requires <type 'datetime.datetime'> object, got %s"
            )
            raise ValueError(tmpl % type(value))
        element = self._get_or_add(prop_name)
        dt_str = value.strftime('%Y-%m-%dT%H:%M:%SZ')
        element.text = dt_str
        if prop_name in ('created', 'modified'):
            # These two require an explicit 'xsi:type="dcterms:W3CDTF"'
            # attribute. The first and last line are a hack required to add
            # the xsi namespace to the root element rather than each child
            # element in which it is referenced
            self.set(qn('xsi:foo'), 'bar')
            element.set(qn('xsi:type'), 'dcterms:W3CDTF')
            del self.attrib[qn('xsi:foo')]