ALTER TABLE `clientes` 
CHANGE COLUMN `created_at` `created_at` TIMESTAMP NOT NULL DEFAULT NOW() ,
CHANGE COLUMN `updated_at` `updated_at` TIMESTAMP NULL DEFAULT NULL ;

ALTER TABLE `concesionarios` 
CHANGE COLUMN `created_at` `created_at` TIMESTAMP NOT NULL DEFAULT NOW() ,
CHANGE COLUMN `updated_at` `updated_at` TIMESTAMP NULL DEFAULT NULL ;

UPDATE `concesionarios` SET `created_at`=NOW(),`updated_at`=NOW();