def METHOD_NAME(self, generator=None):
    """List all the unique timestamps used in the tracks and truth associated
    with a given generator, in order
    Parameters
    ----------
    generator : :class:`~.MetricGenerator`
        :class:`~.MetricGenerator` containing `tracks_key` and `truths_key` to extract
        tracks and truths from :class:`~.MetricManager` to extract timestamps from.
        Default None to take tracks and truths values from first :class:`~.MetricGenerator`
        in `self.generators`.
    Returns
    -------
    : list of :class:`datetime.datetime`
        unique timestamps present in the internal tracks and truths.
    """
    if generator is None:
        generator = self.generators[0]
    timestamps = {state.timestamp
                  for sequence in chain(self.states_sets[generator.tracks_key],
                                        self.states_sets[generator.truths_key])
                  for state in sequence}
    return sorted(timestamps)