def fill_datetime(self):
    """Returns when the slot was filled.

    Returns:
      A datetime.datetime.

    Raises:
      SlotNotFilledError if the value hasn't been filled yet.
    """
    if not self.filled:
      raise SlotNotFilledError('Slot with name "%s", key "%s" not yet filled.'
                               % (self.name, self.key))
    return self._fill_datetime