How to use user() and now() as default values?

I have a table in which I want to keep track of who changes rows and when they change them. Obviously, a timestamp field type will allow me to keep track of the last change to a record, but is there a way to include the username as a default? A default value of user() simply puts 'user()' in the field.

I'd also like to have the current system time inserted when a record is created, but a default value of now() puts 00-00-00 00:00:00 in the field. I find it strange that this is different behavior from the user() default mentioned above.

Thanks,

John Coleman