code: func (b *builtinDurationDurationTimeDiffSig) evalDuration(row chunk.Row) (d types.Duration, isNull bool, err error) {
  	lhs, isNull, err := b.args[0].EvalDuration(b.ctx, row)
  	if isNull || err != nil {
  		return d, isNull, err
  	}
  
  	rhs, isNull, err := b.args[1].EvalDuration(b.ctx, row)
  	if isNull || err != nil {
  		return d, isNull, err
  	}
  
  	d, isNull, err = calculateDurationTimeDiff(b.ctx, lhs, rhs)
  	return d, isNull, err
  }
docs: // evalDuration evals a builtinDurationDurationTimeDiffSig.
  // See https://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_timediff