code: func builtinFromDays(args []types.Datum, ctx context.Context) (d types.Datum, err error) {
  	days, err := args[0].ToInt64(ctx.GetSessionVars().StmtCtx)
  	d.SetMysqlTime(types.TimeFromDays(days))
  	return d, nil
  }
docs: // See https://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_from-days