All you need to do is use REDEFINES to redefine your 9-digit packed variable as an appropriate DATE item. For example:

       01  W-Date                          pic s9(9)
                                               packed-decimal
                                               value 02152012.
       01  W-MMDDYYYY redefines W-Date     format date '%m%d@Y'
                                           usage packed-decimal.

Now you can use the ADD-DURATION function:

           move   function add-duration ( W-MMDDYYYY days 1 )
                          to  W-MMDDYYYY

Tom

01 WS-WRK-DATE FORMAT DATE ‘%Y%m%d’.

01 WS-DAYS PIC S9(05) COMP-3 VALUE 1.

01 WS-DATE PIC X(08).
PROCEDURE DIVISION.
MOVE WS-DATE TO WS-WRK-DATE .
MOVE
FUNCTION ADD-DURATION (WS-WRK-DATE DAYS WS-DAYS) TO
WS-DATE.

If you want to substract 1 day – change WS-DAYS to -1 and use:

MOVE
FUNCTION SUBTRACT-DURATION(WS-WRK-DATE DAYS WS-DAYS) TO
WS-DATE