You could make a call with the document function (in a non-functional
way :) ) to a back end that uses a custom uri resolver that, when
receives a certain href, logs/writes the /real/ current-dateTime and
returns <nothing/>.

That would probably not work. I tested this the other day, just out of
curiosity, calling a script that incremented a number, and found that
the result was cached when using LibXSLT, Saxon and Xalan.

mludwig@forelle:~/Werkstatt/xsl > expand -t2 ~/www/xml.php
<?php
$fnam = 'xml.php.dat';
if (file_exists($fnam)) {
  $i = file_get_contents($fnam);
  file_put_contents($fnam, $i + 1);
}
else {
  $i = 0;
  file_put_contents($fnam, $i);
}
echo "<Int>$i</Int>\n";