Drupal FEED IMPORTER -使用Xpath函数将XML日期时间转换为Unix时间戳

这是我的xml格式。我取了几个值

图片来自Content: substring-before(substring-after(Content， "src=")，' alt=')

如何将XML日期和时间转换为Unix时间戳格式。

<?xml version="1.0" encoding="iso-8859-1"?>
<users>
  <user>
	<id>uuid:59b3ba1f-8e9d-48f5-a73c-f404b3a8a697;id=3831</id>
	<content type="html">
<![CDATA[<img src=http://img2.tvtome.com/i/u/0aa3afb3cbe3468fc6e43e50070b0810.png alt= /><p>"With a Little Help From My Friends" is a song written by the Beatles (and expertly covered by Joe Cocker) and it could easily be the subtitle for the thirty minute comedy,</p>]]>
</content>
    <firstname>Sheila</firstname>    
    <address>2 Good St</address>
	<published>2015-04-02T09:11:44-05:00</published>        
    <contact>
      <phone>12341234</phone>
      <url>http://example.com</url>
      <email>pamela@example.com</email>
    </contact>
  </user>
<users>