gistfile1.php

<?php
/**
 * http://www.php.net/manual/en/datetime.diff.php
 * (PHP 5 >= 5.3.0)
 *
 * Calculate datetime interval.
 */

// Set timezone if need.
date_default_timezone_set('Asia/Taipei');

$open_date = '2013-07-01 12:23:34';
$now = new DateTime('now');
$view_at = new DateTime($open_date);
$interval = $view_at->diff($now);
$view_days = $interval->format('%a');
echo $view_days.' days';

?>