Query MYSQL para seleciona a cada n° rows

gistfile1.txt

SELECT UNIX_TIMESTAMP(dt_criacao) AS 'dt_criacao'
FROM (
    SELECT
        @row := @row +1 AS rownum, dt_criacao
    FROM (
        SELECT @row :=0) r, tb_dados WHERE num_sim = '1' AND dt_criacao BETWEEN '2017-02-01' AND '2017-02-09'
    ) ranked
WHERE rownum %30 =1