docs: def cast_datetime_filter(value):
      """Cast a datetime filter value.
  
      :param value: string representation of a value that needs to be casted to
          a `datetime` object.
  
      """
      if isinstance(value, str):
          dtime = parse_datetime(value)
  
      elif isinstance(value, datetime):
          dtime = value
      else:
          raise ValueError('Received value of type {0}'.format(type(value)))
  
      return dtime.isoformat()