code: func (b *builtinUnixTimestampIntSig) evalInt(row chunk.Row) (int64, bool, error) {
  	val, isNull, err := b.args[0].EvalTime(b.ctx, row)
  	if err != nil && terror.ErrorEqual(types.ErrInvalidTimeFormat.GenWithStackByArgs(val), err) {
  		// Return 0 for invalid date time.
  		return 0, false, nil
  	}
  	if isNull {
  		return 0, true, nil
  	}
  	t, err := val.Time.GoTime(getTimeZone(b.ctx))
  	if err != nil {
  		return 0, false, nil
  	}
  	dec, err := goTimeToMysqlUnixTimestamp(t, 1)
  	if err != nil {
  		return 0, true, err
  	}
  	intVal, err := dec.ToInt()
  	terror.Log(err)
  	return intVal, false, nil
  }
docs: // evalInt evals a UNIX_TIMESTAMP(time).
  // See https://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_unix-timestamp