def grok_datetime(dstring):
    more_formats = ['YYYYMMDDTHHmmss', 'YYYYMMDDTHHmmssZ', 'YYYYMMDD']
    parser = DateTimeParser()
    parser.SEPARATORS += ['']

    # Try to parse datetime string in regular ISO 8601 format
    try:
        return parser.parse_iso(dstring)

    # Fall back to parse datetime string in additional convenience formats
    except arrow.parser.ParserError as ex:
        for format in more_formats:
            try:
                return parser.parse(dstring, format)
            except arrow.parser.ParserError as ex:
                pass

    # Fall back to attempt to parse as relative datetime expression, e.g. "now-10m"
    return get_timedelta(dstring) 

def iso8601_to_dt(iso8601):
    """Given an ISO8601 string as returned by Device Cloud, convert to a datetime object"""
    # We could just use arrow.get() but that is more permissive than we actually want.
    # Internal (but still public) to arrow is the actual parser where we can be
    # a bit more specific
    parser = DateTimeParser()
    try:
        arrow_dt = arrow.Arrow.fromdatetime(parser.parse_iso(iso8601))
        return arrow_dt.to('utc').datetime
    except ParserError as pe:
        raise ValueError("Provided was not a valid ISO8601 string: %r" % pe) 

def strict_parse_args(parser, raw_args):
    """
    Wrapper around parser.parse_args that raises a ValueError if unexpected
    arguments are present.

    """
    args = parser.parse_args()
    unexpected_params = (set(raw_args) - {allowed_arg.name for allowed_arg in
                                          parser.args})
    if unexpected_params:
        raise InputError('Unexpected query parameters {}'.format(
                         unexpected_params))
    return args