para obter um intervalo, como de 30 a 45 minutos atrás, faça assim
SELECT * FROM tbl
WHERE tbl.mydate > DATE(DATE_sub(NOW(), INTERVAL 45 MINUTE))
AND tbl.mydate < DATE(DATE_sub(NOW(), INTERVAL 30 MINUTE));
SELECT * FROM tbl
WHERE tbl.mydate > DATE(DATE_sub(NOW(), INTERVAL 45 MINUTE))
AND tbl.mydate < DATE(DATE_sub(NOW(), INTERVAL 30 MINUTE));