def ipea_date(start=None, end=None):
    """
    Help filter an IPEA time series by date.

    Parameters
    ----------
    start : str
        Start date string.

    End : str
        End date string.

    Returns
    -------
    str
        A string to filter by dates.

    Examples
    --------
    >>> url.ipea_make_dates_query("2019-01-01T00:00:00-00:00", "2019-02-01T00:00:00-00:00")
    '&$filter=VALDATA ge 2019-01-01T00:00:00-00:00 and VALDATA le 2019-02-01T00:00:00-00:00'
    """
    dates = ''
    if start and end:
        dates = f'&$filter=VALDATA ge {start} and VALDATA le {end}'
    elif start:
        dates = f'&$filter=VALDATA ge {start}'
    elif end:
        dates = f'&$filter=VALDATA le {end}'
    return dates
