Here's a simple one I keep in my startup. If the file already exists (for the current day), it will not be copied over again, which is what I wanted.
I should change it to use a copy method other than vl-file-copy, just so I the original file edit date won't be changed, but it's never been a big deal for what this has been there for.
((lambda (/ path)
(if (not (wcmatch (strcase (getvar 'DWGNAME)) "DRAWING*"))
(progn
(vl-mkdir (setq path "c:\\dwgBackup"))
(vl-mkdir (setq path (strcat path "\\" (menucmd "M=$(edtime,$(getvar,date),YYYY.M.DD)") "\\")))
(vl-file-copy (strcat (getvar 'DWGPREFIX) (getvar 'DWGNAME)) (strcat path (getvar 'DWGNAME)))
)
)
)
)