Použijte CURRENT_TIMESTAMP místo CURRENT_DATE() funkce
Zkuste toto:
DROP TABLE IF EXISTS `visitors`;
CREATE TABLE `visitors` (
`id` BIGINT(20) UNSIGNED NOT NULL AUTO_INCREMENT,
`ip` VARCHAR(32) NOT NULL,
`browser` VARCHAR(500) NOT NULL,
`version` VARCHAR(500) NOT NULL,
`platform` ENUM('w','l','m') NOT NULL,
`date` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
PRIMARY KEY (`id`),
UNIQUE KEY `person` (`ip`,`date`)
) ENGINE=INNODB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;