#set($events = $_XPathTool.selectNodes($contentRoot,'//system-data-structure[@definition-path="Event"]'))

$_SortTool.addSortCriterion("start-date", "en", "number", "ascending", "upper-first")  
$_SortTool.sort($events)  

#*

The above would be applied data that looks like this (but where there would be multiple system-page nodes: 

<?xml version="1.0" encoding="UTF-8"?>
<system-data-structure>
    <feed_config>
        ...
        <source_index type="block">
            <content>
                <xhtml-block>
                    <system-index-block current-time="1441638221327" name="events" type="folder">
                        <system-page id="ff878cfc867e488e6adb60030f78641c">
                            <name>2015-09-19-talk-like-a-pirate-day</name>
                            <is-published>true</is-published>
                            <last-published-on>1440706472866</last-published-on>
                            <last-published-by>sujaj</last-published-by>
                            <display-name>Talk like a pirate day</display-name>
                            <path>/events/2015-09-19-talk-like-a-pirate-day</path>
                            <site>CUC15-johns2ja</site>
                            <link>site://CUC15-johns2ja/events/2015-09-19-talk-like-a-pirate-day</link>
                            <created-by>sujaj</created-by>
                            <created-on>1438806150367</created-on>
                            <last-modified-by>sujaj</last-modified-by>
                            <last-modified>1441635743618</last-modified>
                            <system-data-structure definition-path="Event">
                                <start-date>1442635200000</start-date>
                                <end-date>1442718000000</end-date>
                                <title>Talk like a pirate day</title>
                                <content>
                                    <h1>September 19</h1>
                                    <h2>We missed it!</h2>
                                    <p>Not only is September 19th "Talk like a pirate day," but it is also the anniversary of the emoticon 
                                        <strong>:-)</strong>
                                    </p>
                                    <p>The Smiley Emoticon was born September 19, 1982, Thanks to Carnegie Mellon University Professor Scott E. Fahlman.</p>
                                    <p>
                                        <img alt="Emoticon" height="333" src="../images/emoticon.jpg" width="500" />
                                    </p>
                                </content>
                                <i_tags type="block">
                                    <content>
                                        <system-data-structure>
                                            <tag>homepage</tag>
                                            <home type="page">
                                                <content />
                                                <path>/index</path>
                                                <link>site://CUC15-johns2ja/index</link>
                                                <site>CUC15-johns2ja</site>
                                                <name>index</name>
                                                <display-name>Velocity The Basics:</display-name>
                                                <title>Scripting with a $ here and a # to do</title>
                                            </home>
                                        </system-data-structure>
                                    </content>
                                    <path>/_tags/homepage</path>
                                    <site>CUC15-johns2ja</site>
                                    <name>homepage</name>
                                </i_tags>
                                <configs>
                                    <path>/</path>
                                </configs>
                            </system-data-structure>
                        </system-page>
                        ...
                    </system-index-block>
                </xhtml-block>
            </content>
            ...
        </source_index>
        ...
    </feed_config>
</system-data-structure>


*#