code: func (b *builtinToDaysSig) evalInt(row chunk.Row) (int64, bool, error) {
  	arg, isNull, err := b.args[0].EvalTime(b.ctx, row)
  
  	if isNull || err != nil {
  		return 0, true, handleInvalidTimeError(b.ctx, err)
  	}
  	ret := types.TimestampDiff("DAY", types.ZeroDate, arg)
  	if ret == 0 {
  		return 0, true, handleInvalidTimeError(b.ctx, types.ErrIncorrectDatetimeValue.GenWithStackByArgs(arg.String()))
  	}
  	return ret, false, nil
  }
docs: // evalInt evals a builtinToDaysSig.
  // See https://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_to-days