def _format_return_timestamps(self, return_timestamps=None):
        """
        Format the passed in return timestamps value as a numpy array.
        If no value is passed, build up array of timestamps based upon
        model start and end times, and the 'saveper' value.
        """
        if return_timestamps is None:
            # Build based upon model file Start, Stop times and Saveper
            # Vensim's standard is to expect that the data set includes the `final time`,
            # so we have to add an extra period to make sure we get that value in what
            # numpy's `arange` gives us.
            return_timestamps_array = np.arange(
                self.components.initial_time(),
                self.components.final_time() + self.components.saveper(),
                self.components.saveper(), dtype=np.float64
            )
        elif inspect.isclass(range) and isinstance(return_timestamps, range):
            return_timestamps_array = np.array(return_timestamps, ndmin=1)
        elif isinstance(return_timestamps, (list, int, float, np.ndarray)):
            return_timestamps_array = np.array(return_timestamps, ndmin=1)
        elif isinstance(return_timestamps, _pd.Series):
            return_timestamps_array = return_timestamps.as_matrix()
        else:
            raise TypeError('`return_timestamps` expects a list, array, pandas Series, '
                            'or numeric value')
        return return_timestamps_array
Format the passed in return timestamps value as a numpy array.
        If no value is passed, build up array of timestamps based upon
        model start and end times, and the 'saveper' value.