How to get the difference between two dates / time in mysql

CREATE TABLE users (
  users_name varchar(20) NOT NULL,
  users_starttime datetime DEFAULT '00-00-0000 00:00:00' NOT NULL,
  users_endtime datetime DEFAULT '00-00-0000 00:00:00' NOT NULL
);

insert into users (users_name, users_starttime) values('Balaji', NOW());

update users set users_endtime = NOW();

select * from users;

SELECT TIMEDIFF(users_endtime,users_starttime) FROM users;
SELECT TIMESTAMPDIFF(SECOND,users_starttime,users_endtime) FROM users;