CREATE TABLE user(id INT PRIMARY KEY, name VARCHAR, reg_time DATETIME DEFAULT CURRENT_TIMESTAMP);

SELECT * FROM user WHERE ...

    (reg_time >= DATE_SUB(..., INTERVAL ...))
    (reg_time >= NOW() - INTERVAL 24 HOUR)
    (DATEDIFF(NOW(), reg_time) <= ...)
    (TIMESTAMPDIFF(HOUR, NOW(), reg_time) ...)