Updating column on row update

Hi,

This should be simple, but for some reason I'm not quite sure what the
solution is.  I want to be able to update the value of a column for rows
that have been updated.  More specifically, if a row is updated, I want it's
modified_date column to be populated with the current time stamp.  I've
looked at triggers and rules, and it looks like I'd need to create a
function just to achieve this which seems incredibly clumsy and unnecessary.
 Could someone enlighten me?

Thanks

Thom