I would add an id column to this table to add as the primary key, it should be unique, not null and auto increment. As for setting the default value if you can't get it worked out in the db you can do this in the php code for the inserted value. If you would like a full date and time entry for the current time the value you would use in php would look like this:
<?php echo date("Y-m-d H:i:s", time()); ?>