Unpublish date set from a tv's value

<?php

$pdate = $resource->get('publishedon');
$update = $resource->get('unpub_date');
$offset = 86400; // 1 day = 86400 sec

// calculate new unpublish date
$pdate = strtotime($pdate);
$nupdate = $pdate + $offset;

// save new unpublish date into field
$update->set('unpub_date', $nupdate);
$update->save();

<?php
// save new unpublish date into field
$resource->set('unpub_date', $nupdate);
// You shouldn't need this: $resource->save();

if ($resource->get('template') != ##) {
    return;
}

// only run this code for the template with the ID of 6
if ($resource->get('template') != 6) {
	return;
}

// get the publish date
$publish_date = $resource->get('publishedon');

// get the template variable with the number of days after publish date to set unpublish date
$unpublish_days = $resource->getTVValue('unpublish_days');

// calculate unpublish date
$publish_date = strtotime($publish_date);
$offset = 86400 * $unpublish_days;
$new_unpublish_date = $publish_date + $offset;

// save new unpublish date into field
$resource->set('unpub_date', $new_unpublish_date);