def format_modified(self, modified, sep=" "):
        """Format modification date in UTC if it's not None.
        @param modified: modification date in UTC
        @ptype modified: datetime or None
        @return: formatted date or empty string
        @rtype: unicode
        """
        if modified is not None:
            return modified.strftime("%Y-%m-%d{0}%H:%M:%S.%fZ".format(sep))
        return u""
Format modification date in UTC if it's not None.
        @param modified: modification date in UTC
        @ptype modified: datetime or None
        @return: formatted date or empty string
        @rtype: unicode