def get_knot_dates(start_date, knot_idx, freq):
    """
    Parameters
    ----------
    start_date : datetime array
    knot_idx : ndarray
        1D array containing index with `int` type.
    freq : datetime delta

    Returns
    -------
    list :
        list of knot dates with provided start date time and indices
    """
    knot_dates = knot_idx * freq + start_date

    return knot_dates