title: Php Timestamp showing negative values when month change
body: <p>In Php Controller code 'date_added' is already there and i created 'date_sla' for current date. In my out put individually both are working fine but when i use to calculate the difference result is fine for within the month. when month changes it showing negative numbers like (3-1-2017) - (31-12-2016) = -28 days</p>
  
  <pre><code>foreach ($results as $result) {
      $action = array();
  
      $action[] = array(
                  'text' =&gt; $this-&gt;language-&gt;get('text_view'),
                  'href' =&gt; $this-&gt;url-&gt;link('sale/order/info', 'token=' . $this-&gt;session-&gt;data['token'] . '&amp;order_id=' . $result['order_id'] . $url, 'SSL')
              );
  
      if (strtotime($result['date_added']) &gt; strtotime('-' . (int)$this-&gt;config-&gt;get('config_order_edit') . ' day')) {
          $action[] = array(
                      'text' =&gt; $this-&gt;language-&gt;get('text_edit'),
                      'href' =&gt; $this-&gt;url-&gt;link('sale/order/update', 'token=' . $this-&gt;session-&gt;data['token'] . '&amp;order_id=' . $result['order_id'] . $url, 'SSL')
                  );
      }
  
      $this-&gt;data['orders'][] = array(
                  'order_id'      =&gt; $result['order_id'],
                  'customer'      =&gt; $result['customer'],
                  'status'        =&gt; $result['status'],
                  'total'         =&gt; $this-&gt;currency-&gt;format($result['total'], $result['currency_code'], $result['currency_value']),
                  'date_added'    =&gt; date($this-&gt;language-&gt;get('date_format_short'), strtotime($result['date_added'])),
                  'date_sla'    =&gt; date($this-&gt;language-&gt;get('date_format_short'), time()),
                  'date_modified' =&gt; date($this-&gt;language-&gt;get('date_format_short'), strtotime($result['date_modified'])),
                  'selected'      =&gt; isset($this-&gt;request-&gt;post['selected']) &amp;&amp; in_array($result['order_id'], $this-&gt;request-&gt;post['selected']),
                  'action'        =&gt; $action
              );
  }
  </code></pre>
  
  <p>Html code</p>
  
  <pre><code>&lt;td class="right"&gt;&lt;?php echo $order['date_sla'] - $order['date_added']." days";?&gt;&lt;/td&gt;
  </code></pre>