How can I create sequentially numbered invoices in MS Excel for Mac 2008?

[allelala]

I am trying to create an Excel workbook where I have all entire months invoices and each invoice is on a new sheet in the workbook. How can I make it so that the invoice number on each sheet will be incremented by 1.
So that the first invoice on the first sheet might be 2011-001, and the next invoice on the next sheet could automatically be 2011-002. Is this possible?
All the information about sequential numbers that I could find said that this requires writing macros, which I am not able to do (I don't believe) in the MS Excel for Mac 2008 version.

Any ideas how to do this would be greatly appreciated!

 

[ssmed]

Use a formula which references the sheet before

e.g. =Sheet1!A1+1
=Sheet2!A1+1

You could use =INFO("numfile") to get the number of active sheets, but I see problems with this changing existing data.

or have a master sheet at the front with invoice no and contact details on (good for analysis) and reference that.

This is all pretty clumsy though. I would knock up a quick database in Filemaker and manage the whole invoice that way with the potential for analysis and using tables for clients and products/services as well as invoices. They are bound to do a template for this so would be minimal work. Depends on the number of invoices a month I suppose. If a small number manual numbers might be the answer