Excel, creating a running average that automatically progresses

[Destination]

If I have two summated columns, say income and expenditure, which are created from the sum of various data that has been entered.
I can happily summate them, and then subtract one from the other, what I want to do is as each month is entered get a running total of the expenditure versus income over the past twelve months, preferably in some automated fashion, rather than having to resummate and then average the last 12 cells every single month.

Any easy method for this?

 

[Grrrrr]

If you want to, for example, sum expenditure from the past 12 months you can do this:

=SUMIF( 'Range with dates' , ">" & NOW() - DATE(1,0,0) , 'Range with expenditure' )

NOW() returns todays date
DATE(year,month,day) returns the datevalue for a given date, in this case 1 year

so..

">" & NOW() - DATE(1,0,0) tells the function to sum all days 'greater than' 12 months ago.

Does that help?

 

[Destination]

Sort of yes
I have columns of summed items, 2 of them, each produced each month, to do with that months figures.

What I am wondering is, if I can easily create a formula, that takes the most recent 12 figures in each column, as in 12 months income, and 12 months expenditure, and output it to a fresh column beside the two existing ones.
Its easy to add items to existing column, but I would like something that would autoupdate, so it just sums 12 items and outputs, without me having to alter the cell formula each month?
A progressive formula I suppose.

 

[Grrrrr]

It's a bit tricky to understand written out in words. Can you upload a spreadsheet for us to see? (with personal data removed)