How to get datetime annotation from Blueprint Form in Template?

[ICT Gemeente Tynaarlo]

I'm trying to create a Blueprint Form in combination with an XML Template to create pages.

I already do have a form in .soy format, which includes a Date-Picker to get the date from a form:

simplebp.soy:

..
<form action="#" method="post" class="aui">
<fieldset>
<div class="field-group">
<label for="vdatum">{getText('my.blueprint.form.label.title.vDATUM')}</label>
<input class="aui-datepicker" name="vDATUM" id="vdatum" type="date" size="10" autocomplete="off">
</div>
..

The datepicker is working fine, and when calling vDATUM from an XML-template it is filled with the chosen date:

myTemplate.xml:

..
<tr>
<th colspan="1">Datum</th>
<td colspan="1">
<div class="content-wrapper">
<p><at:var at:name="vDATUM" at:rawxhtml="true"/></p>
</div>
</td>
</tr>
..

How can I change the template so, that the date is annotated like the Confluence DateTime-format (the "double // method) for use in a Page Properties Index page?

Syntax:

<time datetime="2019-05-23">

NOT WORKING:

<time datetime="<at:var at:name="vDATUM" at:rawxhtml="true"/>">

Can anyone help me out?