Whether to use or not to use: DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP

In one Database table, created_at & updated_at are defined as: created_at timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP updated_at timestamp NOT NULL DEFAULT '0000-00-00 00:00:00' COMMENT 'Updated At', And in another database table, it's defined as: created_at timestamp NOT NULL DEFAULT '0000-00-00 00:00:00' COMMENT 'Created At', updated_at timestamp NOT NULL DEFAULT '0000-00-00 00:00:00' COMMENT 'Updated At', created_at -> stores datetime when the data is created. updated_at -> stores the datetime when data is updated. Which definition among above two would you prefer & why?