code: func (b *builtinPeriodDiffSig) evalInt(row chunk.Row) (int64, bool, error) {
  	p1, isNull, err := b.args[0].EvalInt(b.ctx, row)
  	if isNull || err != nil {
  		return 0, isNull, err
  	}
  
  	p2, isNull, err := b.args[1].EvalInt(b.ctx, row)
  	if isNull || err != nil {
  		return 0, isNull, err
  	}
  
  	return int64(period2Month(uint64(p1)) - period2Month(uint64(p2))), false, nil
  }
docs: // evalInt evals PERIOD_DIFF(P1,P2).
  // See https://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_period-diff