How to express with SQL statement: create a new field so that the current time will be automatically marked each time a new record is inserted?

  mysql, question

I found this expression on the Internet and implemented the above functions on mysql (version 5.7) brought by local wamp.

alter table account.users add column reg_time datetime not null default now();

But on the server side, I use the same SQL statement

alter table users add column reg_time datetime not null default now();

But prompt: invalid defaultvalue for ‘reg _ time’
The mysql version of the server is 5.0
What should I do?

ALTER TABLE `users`
 ADD COLUMN `reg_time`  timestamp NOT NULL ON UPDATE CURRENT_TIMESTAMP