AutoCAD Visual LISP / AutoLISP :: Expire Date For A Function

I want to define en expiration date in registry, after that date the some LISP programs does not allow to run. I think that suitable commands are SetEnv and GetEnv.

define a ExpDt variable that contains Expiry Date as a string. And add some statements inside of the LISP program to call this variable and check it with today's date.

I should define once time in autocad:

(setenv "expdt" "130101"); YYMMDD date for validation period (2013.01.01)

and check this every time that a lisp program should be execeute:

(getenv "expdt") ; if today's date(with YYMMDD format) ">" expdt then lisp program doen not allow to run and should be halted.

for e.g. on 2013.02.28: "130228" > "130101" ==> so the lisp function should be halted and will not work!

(without any alert or message)

a sample list function: (how can implement on this lisp function?)

(defun c:3()
(setq w t)
(while w
(setq a (getpoint "
pick point:"))
(setq p1 (list (- (car a) 0.5)(- (cadr a) 0.5)))
(setq p2 (list (+(car a) 0.5)(+(cadr a) 0.5)))
(command "erase" "w" p1 p2 "")
(command "INSERT" "*3" a "" "" "0")
)
(if (= a nil)(setq w nil))
)