create a formula to know last months column character or number

[bobh]

Hi All,
I have a need to figure out which column to use in a formula and I'm
not able to figure it out so hopefully someone here can help me.

what I have/know in this company the fiscal yr starts oct 1st, ends
sept 30th,
on the spreadsheet I have

starting column C thru N, row 6
Oct, Nov, Dec, Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep

on the 1st business day of each month the prior months data is added
so this being june right now the data going across columns c thru n
would look like;

12, 45, 36, 73, 22, 71, 29, 55, 0, 0, 0, 0
23, 32, 43, 56, 65, 57, 39, 0, 0, 0, 0, 0
19, 75, 16, 33, 82, 41, 69, 37, 21, 0, 0, 0

in column Q of each row I'm looking to modify the formula that will
determine
that last month(this being june) was May and May is column J and then
change itself to use column J & rownbr & -P & rownbr

so, if in column Q I had from last months run I7-P7 for all the rows
and this now being
june the formula should change itself to J7-P7 and then copy down thru
all the rows and recalculate

hope I made sense
bobh.