Trevor,

Each of the functions used, such as YEAR() and MONTH() is a basic Excel

function, and each is described in Excel help, with examples, and usually

with "See also" links to similar functions.

To modify an old quote, most of us here are not in the business of just

giving you fish, but we are happy to teach you *how* to fish, in other words

we are happy to help you to help yourself if you wish to learn. If you

don't understand what part of a formula is doing, you can always break it up

into manageable chunks to see how each part behaves. Hence you can put

12*(YEAR(A1)-2007)+MONTH(A1) in a cell, and see how it reacts to your

changing input.