code: func builtinDateFormat(args []types.Datum, ctx context.Context) (types.Datum, error) {
  	var d types.Datum
  	date, err := convertToTime(ctx.GetSessionVars().StmtCtx, args[0], mysql.TypeDatetime)
  	if err != nil {
  		return d, errors.Trace(err)
  	}
  
  	t := date.GetMysqlTime()
  	str, err := t.DateFormat(args[1].GetString())
  	if err != nil {
  		return d, errors.Trace(err)
  	}
  	d.SetString(str)
  	return d, nil
  }
docs: // See http://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_date-format