The following was taken from a message posted by Bat Lang in the BATPOWER
echo.
 ST> Could someone please point out to me how to rename my file extensions
 ST> to the Julian date.  Such as having my batch automatically rename my
 ST> daily backup from backup.arj to backup.168 for todays date.

The following can be found through BFDS, which will satisfy the above need.
JULIAN2.ZIP      5582  01-04-95  JULIAN returns the day of year(Julian)
                                 using the system date, and inserts it into the
                                 environment Variable 'JULIAN'.
So your batch file would run JULIAN, then there is a ENVAR called JULIAN
whose value is the current Julian date.  Then you would follow that with
a statement like:  Rename backup.ARJ *.%julian%.  So your batch file
would only *need* to have these two lines:

Julian
ren backup.arj *.%julian%

