The datetime the xml file was last modified.

def timestamp_feature(catalog, soup):
    """The datetime the xml file was last modified.
    """
    # there's really no "time created", we're using the date the courses are listed for...
    epoch = 1318790434
    catalog.timestamp = int(float(soup.title.text)) + epoch
    catalog.datetime = datetime.datetime.fromtimestamp(catalog.timestamp)
    logger.info('Catalog last updated on %s' % catalog.datetime)