Mutating from trigger

Okay,

I'm not too sure about google and what is bringing back, so I will ask probably a common question here.

A trigger cannot update a column in the row that is currently being updated?

Then, how does one use the database to set a specific field when an application is executing the update. We are trying not to use the application to take care of modification dates, but instead of, have the database do it.

-- 
Galen Boyer
I like to skate on the other side of the ice.