Insert Date/Time Stamp

Hi,

I have a form where a user will update a field labeled status to one of
several options in a drop down list. As soon as the user populates their
appropriate selection, I want a colum in my underlaying table to populate the
current date/time. I dont necessarily care if the date/time is displayed on
my form but want a record of when the selection was made....nothing more.
What are my steps for doing so?? Can anyone help with this?