def convert_to_culled_timestep(self, timestep=<NUM_LIT:1>):
valid_s = self.header.analysis_period.VALIDTIMESTEPS.keys()<EOL>assert timestep in valid_s,'<STR_LIT>'.format(timestep, valid_s)<EOL>new_ap, new_values, new_datetimes = self._timestep_cull(timestep)<EOL>self.header._analysis_period = new_ap<EOL>self._values = new_values<EOL>self._datetimes = new_datetimes<EOL>
Convert this collection to one that only has datetimes that fit a timestep.