Thread: cumulative monthly figures - whats the best approach??

I'm quite new to excel and am trying to modify a spreadsheet that I have to use at work so that it will calculate the value of overdue invoices each month. I've created a cell called current month which is used as the startpoint.

the logic is as follows:

current month =May - overdue amount any value in the due colume in April
current month=June - overdue amount any value in the due columns of April and May.
and so on until March of the following year

I've tried nested IF forumulas but this restricts me to 7 nested IF statements but I need much more than 7 (as there are 12 months in the year) and have played with sum if formula but I'm too much of a novice to work out a workaround that lets me exceed the 7 nested function rule.....

anyone got any suggestions as to the the best way to progress this? I've attached the file to demonstrate what I'm trying to do....can anything help..

I keep a running totla of various variables in my pay worksheet
for example accumulated gratuities for the year

=sum(pay1:pay2!a1)

the sum of the amount in cell a1 on sheet pay1 and the sum of that cell in pay2 plus that cell in pay1 and so on.
My problem is trying to get the sheet name to change automatically without manually changing it everytime I create a new pay period

I have used the formula a number of times in each sheet and is a pain going back to change it.