Calc diff of data between two time periods

[Dan]

Example table headers:

FcstMonth | Brand | DateofData | Qty | Market

Note: "Date of Data" is the month of the year
while "FcstMonth" is the month in which the DateofData
was forecasted).

With a cross-tab query, I have queried the data so that
the
Qty field is the "Value"; the DateofData field is column
heading, while the remaining items are Row headings.

What I have in the report thus far:
JAN FEB MAR ... DEC TOTAL
FcstMonth 1
Brand 1 10 xx
Brand 2 xx xx
Brand 3 xx xx
Total xx xx

FcstMonth 2
Brand 1 12 xx
Brand 2 xx xx
Brand 3 xx xx
Total xx xx

What I am attempting to do:
The 2 "FcstMonth" sections represent 2 differnet
forecasting periods with the intention of finding out the
change in forecast for each month across the top. In
other words, FcstMonth 1 is 10 and FcstMonth 2 is 12,
giving a difference (increase in this case) of 2.

What I don't know how to do:
How to put the forula together that does the month that
displays the change between FcstMonth 1 and FcstMonth 2.
The challenge as I see it is that both FcstMonth's are in
the same column of data. There is just one text box for
each monthly datapoint in the report, I don't understand
how I would calcualte the change.


Any help appreciated,
Dan