code: def convert_to_culled_timestep(self, timestep=1):
          
          valid_s = self.header.analysis_period.VALIDTIMESTEPS.keys()
          assert timestep in valid_s, \
              'timestep {} is not valid. Choose from: {}'.format(timestep, valid_s)
  
          new_ap, new_values, new_datetimes = self._timestep_cull(timestep)
          self.header._analysis_period = new_ap
          self._values = new_values
          self._datetimes = new_datetimes
docs: Convert this collection to one that only has datetimes that fit a timestep.