code: func (b *builtinSubDateAndDurationSig) evalString(row chunk.Row) (string, bool, error) {
  	arg0, isNull, err := b.args[0].EvalDuration(b.ctx, row)
  	if isNull || err != nil {
  		return "", isNull, err
  	}
  	arg1, isNull, err := b.args[1].EvalDuration(b.ctx, row)
  	if isNull || err != nil {
  		return "", isNull, err
  	}
  	result, err := arg0.Sub(arg1)
  	return result.String(), err != nil, err
  }
docs: // evalString evals a builtinSubDateAndDurationSig.
  // See https://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_subtime