This question has been answered by BobRay. See the first response.
<p>We will be closed until [[+back_date]]</p>
<?php
$closed = $modx->getOption('closed_until');
if (empty($closed)) {
$output = '';
} else {
$output = $modx->getChunk('closedUntil', array('back_date' => $closed));
}
return $output;$closed = $modx->getOption('closed_until');
return empty($closed)? '' : $modx->getChunk('closedUntil', array('back_date' => $closed));