the format strings which are allowed.

    Parsing is tried using each format, in order, and the first format which
    parses successfully is used.

    :param formats: A list or tuple of date format strings, in the order in
                    which they should be tried. Defaults to
                    ``'%Y-%m-%d'``, ``'%Y-%m-%dT%H:%M:%S'``,
                    ``'%Y-%m-%d %H:%M:%S'``.
    r