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

    Calculates the start point :attr:`t_start` from the three parameter
    :attr:`t_stop`, :attr:`num_bins` and :attr`binsize`.

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

    Returns
    -------
    t_start : quantities.Quantity
        Starting point calculated from given parameter.
    """
    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