Answers for "set default value for datetime column in mysql"

mysql default timestamp value to be current timestamp

CURRENT_TIMESTAMP

You can use two timestamp in one table. For default, use DEFAULT field first 
and then the rest timestamp fields.

Below query should work.

CREATE TABLE myTable 
(
 id INT,
 date_registered TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP, 
 date_validated TIMESTAMP
);

set default today mysql

ALTER TABLE table
ADD column DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP;