def _calc_tstart(num_bins, binsize, t_stop):
    """
    Calculates the start point from given parameters.

    Calculates the start point `t_start` from the three parameters
    `num_bins`, `binsize`, `t_stop`.

    Parameters
    ----------
    num_bins : int
        Number of bins
    binsize : pq.Quantity
        Size of Bins
    t_stop : pq.Quantity
        Stop time

    Returns
    -------
    t_start : pq.Quantity
        Starting point calculated from given parameters.
    """
    if num_bins is not None and binsize is not None and t_stop is not None:
        return t_stop.rescale(binsize.units) - num_bins * binsize
