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
