def _format_hours(values: list) -> list:
    """Return a list of xx:00 formated time strings.

    Returns a list xx:00 formated time strings from a list of input hours.
    Invalid iput hours (e.g. outside of 0-23) will raise an exception.

    Parameters
    ----------
    values: list(int)
        List of month numbers (0-23).

    Returns
    -------
    list(str)
        List of xx:00 formatted time strings (e.g. ['00:00', '01:00', ...,
        '23:00']).
    """
    returnlist = []
    for value in values:
        assert (int(value) >= 0), (
            'invalid value specified for hours: {}'.format(value))
        assert (int(value) <= 23), (
            'invalid value specified for hours: {}'.format(value))
        returnlist += ["{}:00".format(str(value).zfill(2))]
    return returnlist