code: def _datetime_from_json(value, field):
      
      if _not_null(value, field):
          if "." in value:
              # YYYY-MM-DDTHH:MM:SS.ffffff
              return datetime.datetime.strptime(value, _RFC3339_MICROS_NO_ZULU)
          else:
              # YYYY-MM-DDTHH:MM:SS
              return datetime.datetime.strptime(value, _RFC3339_NO_FRACTION)
      else:
          return None
docs: Coerce 'value' to a datetime, if set or not nullable.
  
      Args:
          value (str): The timestamp.
          field (.SchemaField): The field corresponding to the value.
  
      Returns:
          Optional[datetime.datetime]: The parsed datetime object from
          ``value`` if the ``field`` is not null (otherwise it is
          :data:`None`).