How do I update a date field within a form when a user modifies another field (example approves the form request)

How do I update a date field within a form when a user modifies another field (example approves the form request)

Totally confused!  I'm trying to declare a variable and fetch the most recent record when field is updated.  I don't know the fetch arguments in order to update the particular field. 
 
(My application has a form to request $ for loans...I need a date stamp when the user approves or rejects it.)