def time_indices(npts, dt, start, end, index):
    """
    Determine the new start and end indices of the time series.

    :param npts: Number of points in original time series
    :param dt: Time step of original time series
    :param start: int or float, optional, New start point
    :param end: int or float, optional, New end point
    :param index: bool, optional, if False then start and end are considered values in time.
    :return: tuple, start index, end index
    """
    if index is False:  # Convert time values into indices
        if end != -1:
            e_index = int(end / dt) + 1
        else:
            e_index = end
        s_index = int(start / dt)
    else:
        s_index = start
        e_index = end
    if e_index > npts:
        raise exceptions.ModelWarning("Cut point is greater than time series length")
    return s_index, e_index