Rolling Periods

[mccloud]

I have a form that a user can enter the number of periods to review up to 24.
Current period (Nov 2009) is 23. How can I calculate the correct periods to
sum.
My statement would work something like this; (number of periods to review
Example 4) - (current period Example 23) then add periods (19+20 +21+22).
Easy enough but how do I handle when the period rolls back to 1. I thought
about using a table but the number of variables would make this large. Is
there a betterway?
Thanks

 

[KARL DEWEY]

One thought is to have another field to go along with the period - era.