it should be something like this:
src/components/form.lisp:
(defmethod (setf value) (new-value (field date-field))
(multiple-value-bind (s m h day month year)
(decode-universal-time new-value)
(declare (ignore s m h))
(setf (value (slot-value field 'year)) year)
(setf (value (slot-value field 'month)) month)
(setf (value (slot-value field 'day)) day)))