<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def convert_to_culled_timestep(self, timestep=1):
 """Convert this collection to one that only has datetimes that fit a timestep."""

        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