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 = "http://xml.juniper.net/junos/*/junos";
ns xnm = "http://xml.juniper.net/xnm/1.1/xnm";
ns jcs = "http://xml.juniper.net/junos/commit-scripts/1.0";

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

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