The simplest way to see what XML results are returned from jcs:invoke() is to run the command manually with | display xml appended. When jcs:invoke() returns its XML result, it assigns the child element of <rpc-reply> to the $results variable. In this case it is <xnm:error>.

version 1.0;

ns junos = "*/junos";
ns xnm = "";
ns jcs = "";

import "../import/junos.xsl";

match / {
    <op-script-results> {
        var $results = jcs:invoke( "get-system-uptime-information" );
        <output> $results/system-booted-time/date-time;