Have a look at the JULIAN.lsp code. There's nothing magic in there, in fact it does a VERY thorough job of describing HOW it works along with references. It wouldn't be too difficult to duplicate what it does.......especially since you only really need a small bit of it.
On second thought, here's one I put together a few years ago that will handle another type of date (that which MS File System Objects use).....
;;function to convert a computer standard serial date,
;; such as what TDCREATE returns, to a calendar date.
;; If FSO? is not nil then the input date is treated as
;; a MS date as returned by the FileScriptingObject
;; Jeff Mishler c. 2003
(defun ser2cal (serdate FSO?)
(if FSO?
(setq serdate (+ serdate 2415019))
)
(menucmd
(strcat "M=$(edtime,"
(rtos serdate 2 8)
",DDD\",\" DD MON YYYY hh:mm)"));;change as desired, see ACAD help for syntax
)
Then in use similarly to my previous post:
(setq cdate (getvar "date"))
(princ "\nStart date: ")
(princ (ser2cal cdate nil))
(setq futuredate (+ cdate 30))
(princ "\nEnding date: ")
(princ (ser2cal futuredate nil))
This returns:
Start date: Sat, 01 Jul 2006 16:19
Ending date: Mon, 31 Jul 2006 16:19