def to_iso_date(date, default=None):
<EOL>if not is_date(date):<EOL><INDENT>return default<EOL><DEDENT>if isinstance(date, (DateTime)):<EOL><INDENT>return date.ISO8601()<EOL><DEDENT>return date.isoformat()<EOL>
ISO representation for the date object

    :param date: A date object
    :type field: datetime/DateTime
    :returns: The ISO format of the date
    :rtype: str